• Most unicellular organisms are of microscopic size and are thus classified as microorganisms, for example, Phytoplankton and protozoa range from about 0.001 mm to about 0.25 mm.
• Most of the multicellular organisms and vertebrates are visible to the naked eye, which means that it is possible to view them without using the microscope.
  So, option A is the correct answer.
  

A neuron is a nerve cell that is the basic building block of the nervous system. Neurons are similar to other cells in the human body in a number of ways, but there is one key difference between neurons and other cells. Neurons are specialized to transmit information throughout the body. Nerve cell or neuron possess a cell body with a long, thin hair like axon. A nerve cell can be up to a metre long. 

• An erythrocyte is a cell containing large amounts of hemoglobin and specialized for oxygen transport. In mammals, erythrocytes are round and biconcave in shape. 
• They are devoid of nucleus and other membrane-bound organelles. They derive energy through glycolysis and serve as tiny packets of hemoglobin.

The cell is the structural unit of life with various shapes and size. The size of the cell ranges from 0.5 micrometers to 100 micrometers. Mycoplasma gallicepticum, a parasitic bacterium is thought to be the smallest known organisms with the size from 0.2 to 0.3 micrometer, capable of independent growth and reproduction. Later, the name for mycoplasma was pleuropneumonia-like organisms or PPLO.

The tissues which cover or protect the body are called epithelial tissues. Cells of the epithelial tissues are tightly packed and have a small amount of cementing material in between them. These are small cells covering the organs and cavities of the body.  
Mast cells are a type of cells belonging to areolar connective tissues. It fills the space around the organs. 
Kupffer cells are star-shaped macrophages present in the liver lining. 
Nerve cells possess a cell body and one or many thin fiber-like axons. Some nerve cells possess axons as long as 1 meter and thus, are the largest cell in the human body. It is also known as the longest cell in the human body. Thus, the correct answer is option A.

The ostrich egg is the largest cell known. These eggs are 15 cm long and 13 cm wide. 

• The cell is the structural unit of life. Unicellular organisms are made up of one cell that performs all the functions. Its size is 0.5-10 micrometers, whereas, in multicellular organisms, cell size is 10-100 micrometers.

Sclerenchyma, in plants, support tissue composed of any of various kinds of hard woody cells. Mature sclerenchyma cells are usually dead cells that have heavily thickened secondary walls containing lignin. The cells are rigid and nonstretchable and are usually found in nongrowing regions of plant bodies, such as the bark or mature stems. Sclerenchyma is one of the three types of ground, or fundamental, tissue in plants.Sclerenchyma fibers can be found below the epidermis of monocotyledons, organized in two or three layers of cells. They are known as cortical fibers. Thus correct option is D.

Human sperm consists of a head and tail, the head carrying the genetic material and tail has mitochondria which derives the movement of sperm. It is very small in size (0.05mm) and a drop of serum contains millions of sperms. The human egg consists of genetic material along with core cytoplasm surrounding nucleus that has all the cellular components such as ribosomes, RNAs which are essential for protein synthesis and which ultimately gives rise to the whole body of an organism. Human egg measures 2cm against human sperm 0.05mm.

The egg size of Ostrich egg is 15cm, duck egg is typically larger than hen eggboth of which are roughly between 3-10cm. Human female egg is very small about 1-2cm and hence the smallest egg among all other options.

Acetabularia is a single-celled eukaryotic photosynthetic green alga. Although it is single cells it has a complex structure with size varying from 0.5-10cm. It is divided into three anatomical parts with basal rhizoid like structure resembling roots, long stalk-like structure and top umbrella that may fuse to form the cap.  

If the appearance of complexity is concurrent with evolution then, human beings are more evolved and complex than bacteria. Humans are the multicellular organism with the well-developed brain. Bacteria is the primitive unicellular organism.

Safranin is a biological stain composed of azonium compounds, which is utilized to study histology and cytology. In particular, safranin stains lignified tissues like xylem even in the presence of cellulose particles. Fluorescent safranin staining is useful to stain the cell wall of woody plants and produce green or yellow fluorescence in the secondary cell wall and red or orange fluorescence in the middle lamella region under a fluorescent microscope. 

All cells usually have a fixed life span. The cells divide for a certain period of time, after which they die. This has been explained by the phrase "programmed cell death". The lifespan of each type of the body cells differ and it depends on the function it is destined to perform. Neurons are long lived and are generally non-mitotic. Memory T cells and memory B cells can provide long lasting immunity from exposure to antigens, they too are long lasting but their life span is less than that of a neuron.

PPLO is pleuropneumonia-like organisms. It has the diameter of 0.1 micrometer. It is the smallest known cell. 

The cell is the structural unit of life and its size ranges from 0.5 micrometers to 100 micrometers. An egg of ostrich is a single-celled structure and know as the largest cell in the world with the size of 130 mm-170 mm. Thus, the correct answer is option C.