A retrovirus has been identified as a single-stranded positive sense RNA virus which has an intermediate DNA that helps it target the host cell as an obligate parasite. This virus reverses the usual pattern and uses its transcriptase enzyme to produce DNA. The transmission of these viruses been generally cell-to-cell. In human beings, cancer can be triggered by proto-oncogenes present in the retrovirus which are mistakenly incorporated into proviral DNA or by the disruption of the cellular proto-oncogenes

• Cancer is a class of diseases characterized by out-of-control cell growth. There are over 100 different types of cancer, and each is classified by the type of cell that is initially affected.
  
• Cancer harms the body when damaged cells divide uncontrollably to form lumps or masses of tissue called tumors. Tumors can grow and interfere with the digestive, nervous, and circulatory systems, and they can release hormones that alter body function. Tumors that stay in one spot and demonstrate limited growth are generally considered to be benign.
  

 Polycythemia is a disease state in which the hematocrit (the volume % of RBC in the Blood) is elevated.It can be due to an increase in the number of RBC ("absolute polycythemia") or to a decrease in the volume of plasma ("relative polycythemia").

'Philadelphia chromosome' is found in the patient suffering from leukaemia. The main cause is a genetic abnormality in chromosome 22 of leukaemia cancer cells. The chromosome is short due to a reciprocal translocation between chromosome 9 and chromosome 22 and contains a fusion gene called BCR-ABL1 which is juxtaposed onto the BCR gene of chromosome 22, coding for a hybrid protein, a tyrosine kinase signalling protein which causes rapid multiplication of the cell.

Oncogenes are genes that has the potential to cause cancer.
Estrogens are a group of hormones that are important for female sexual and reproductive development.
Carcinogens are substances, radionuclide or radiation that is an agent directly involved in causing cancer.
Therefore, the correct answer is option D.

The spread of tumour cells through vascular system (blood, lymph) from the site of formation to other parts of the body is known as metastasis.

The ozone layer is present in the stratosphere above the Earth. The layer of ozone is responsible for the absorption of the harmful ultraviolet radiations from the solar radiations. These radiations are prevented from entering the Earth's atmosphere to prevent the development of skin cancer. The radiations can also damage the eye and other exposed parts of the body. 

Carcinogen is a substance or agent that can cause cells to become cancerous by altering their genetic structure so that they multiply continuously and become malignant. Asbestos, DDT, and tobacco smoke are examples of carcinogens.

Cancer arises from the abnormal, purposeless, and uncontrolled division of cells that then invade and destroy the surrounding tissues.

Cell cycles are regulated by cyclins and Cyclin dependent kinases (CDK). In cancer cells this regulatory mechanism is lost, thus the cells divide or proliferate in an uncontrolled manner after a certain stage, they produce growth factors and respond to them in an autocrine manner. Since, they are the body cells they are not destroyed by the immune system. They are capable of separating from the parent tumor and establishing else where in the body, a process called metastasis.

Saccharin is a artificial sweetener. It is thermostable and remains inert at high temperature. It was discovered in 1879. It helps to control weight and its good for diabetes patients. It was used as sugar substitute in place of sucrose. It has been discontinued because it was considered a carcinogenic agent by FDA.

p53 is a protein is known as  "guardian of genome" because it regulates cellular apoptosis and cellular growth to maintain the integrity of the human genome. It is also known as policeman of oncogenes because it suppresses the mutation and prevents oncogenesis.

Tumour promotion is a process in which an existing tumour is stimulated to grow. The tumour promoter is defined as a chemical, a complex of chemicals, or a biological agent that promotes a later stage of carcinogenesis, called tumour promotion by altering transcription (expression of the genetic information) rather than altering the structure of DNA. A tumour promoter is not involved in the formation of a new tumour.

Lung cancer, also known as lung carcinoma is a malignant lung tumour characterized by uncontrolled cell growth in tissues of the lung. Most cancers which start in the lungs, known as primary lung cancers are carcinomas. Chromium fluoride is a carcinogenous hexavalent chromium which is found in the tap water. It increases the risk of lung cancer, kidney cancer, intestinal cancer etc.

An oncogene is a gene that has the potential to cause cancer. In tumour cells, they are often mutated or expressed at high levels. Activated oncogenes can cause those cells designated for apoptosis (programmed cell death) to survive and proliferate instead. Most oncogenes began as proto-oncogenes which are normal genes involved in cell growth and proliferation or inhibition of apoptosis.

A carcinogen is any substance or radiation that promotes carcinogenesis, the formation of cancer.  Carcinogens may increase the risk of cancer by altering cellular metabolism or damaging DNA directly in cells. N-nitrosodimethylene causes liver cancer, whereas some of the tobacco-specific nitrosamines cause lung cancer. Volatile N-nitrosamines induce tumours in a variety of human organs, including the tongue, oesophagus, lung, pancreas, liver, kidney and bladder. 

Cancerous cells spread to other parts of the body as it is able to break away from the original (primary) tumour and get into the bloodstream or lymph system, which can carry them to another part of the body. Cancerous cells also spread through metastasis. Metastasis is the development of secondary malignant growths at a distance from a primary site of cancer. When tumour cells metastasize, the new tumour is called a secondary or a metastatic tumour, and its cells are similar to those in the original or primary tumour.

• Cervical cancer is cancer arising from the cervix. It is due to the abnormal growth of cells that have the ability to invade or spread to other parts of the body. It is caused by Human Papillomavirus
  
• Initially, no symptoms are seen. Later symptoms may include abnormal vaginal bleeding, pelvic pain, or pain during sexual intercourse.
  
• Hence Cervical cancer is caused by Human Papillomavirus.
• So, the correct answer is 'Human Papillomavirus'.

In nuclear leakage, apart from the damage caused by fires and explosions, accidents also release radioactive materials which can cause radiation sickness. The radiation possesses carcinogenic potential. It will induce cancer in tissues of most species at all ages including the fetus. Nuclear waste is hazardous for tens of thousands of years.

According to World health organisation, prolonged human exposure to solar UV radiation may result in acute and chronic health effects on the skin, eye and immune system. 

Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. A proto-oncogene is a normal gene that could become an oncogene due to mutations and cancer. Proto-oncogenes code for proteins that help to regulate cell growth and differentiation. A tumour suppressor gene or antioncogene is a gene that protects a cell from one step on the path to cancer. Cancer develops due to inactivation of a tumour suppressor gene.

Bladder cancer is any of several types of cancer arising from the tissues of the urinary bladder. Symptoms include blood in the urine, pain with urination, and low back pain. Thirty per cent of bladder tumours probably results from occupational exposure in the workplace to carcinogens such as benzidine. 2-Naphthylamine and 4-aminobiphenyl which is found in cigarette smoke and has shown to increase bladder cancer risk.

The International Agency for Research on Cancer (IARC) part of the World Health Organisation announced that it had reclassified diesel exhaust as a 'definite carcinogen in highest category (Category 1). When diesel burns inside an engine it releases two potentially cancer-causing things: microscopic soot particles, and chemicals called polycyclic aromatic hydrocarbons or PAHs. There are three possible ways these can cause cancer. The microscopic particles making up diesel exhaust particulate matter are less than one-fifth the thickness of a human hair. They are small enough to penetrate deep into the lungs, causing respiratory disease. Inhaled PAHs could directly damage the DNA in the cells of our lungs leading to cancer. Secondly, the soot particles can get lodged deep inside the lungs, causing long-term inflammation, and thirdly this can increase the rate at which cells divide. So if any nearby lung cells pick up random mutations, this inflammation could make them more likely to grow and spread. 

So, the correct  is 'a-iv, b-i, c-ii, d-iii'.

Ulceration of the gastrointestinal mucosa is caused by disruption of the mucus layer on the gastric epithelial cells. It is wrong to believe that eating acidic foods may lead to development of peptic ulcers. Ulcer may be viewed as an extension of the cell damage found in acute gastritis. The most common sites for ulcers are the stomach and the first few centimetres of the duodenum. The main factors behind ulcer are:
(1) Reduced blood supply (Reduced blood flow and ischaemia may be caused by excessive cigarette smoking and stress, either physical or mental).
(2) Reduce mucus secretion (The composition and the amount of mucus may be altered, e.g., by regular and prolonged use of aspirin and other anti-inflammatory drugs by the reflux of bile acids and salts in chronic gastritis) and
(3) Raised levels of steroid hormones, e.g., in response to stress or when they are used as drugs in chronic gastritis by irradiation and the use of cytotoxic drugs).

Aflatoxins are naturally occurring mycotoxins that are produced by Aspergillus flavus and Aspergillus parasiticus, species of fungi. Aflatoxins are toxic and among the most carcinogenic substances known. After entering the body, aflatoxins may be metabolized by the liver or hydroxylated to become the less harmful aflatoxin M1. Grains such as corn, wheat, and rice, as well as nuts and legumes, are often contaminated with molds, often as a result of poor growing conditions, substandard or extended storage. Molds called aflatoxins can easily grow and produce a very potent carcinogen.