Micro-injection is a method used to Inject a recombinant DNA into the nucleus of an animal cell. This is the only way to introduce alien DNA into host cells. 

Alternative selectable markers have been developed which differentiate recombinants from the non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate. In this, a recombinant DNA is inserted within the coding sequence of an enzyme, ( -galactosidase. This results into inactivation of the enzyme, which is referred i to as insertional inactivation. The presence of a chromogenic j substrate gives blue coloured colonies if the plasmid in the bacteria does not have an insert. Presence of insert results into insertional inactivation of the -galactosidase and the colonies do not produce any colour, these are identified as recombinant colonies.

Alternative selectable markers have been developed which differentiate recombinants from the non-recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate. In this, a recombinant DNA is inserted within the coding sequence of an enzyme, ( -galactosidase. This results into inactivation of the enzyme, which is referred i to as insertional inactivation. The presence of a chromogenic j substrate gives blue coloured colonies if the plasmid in the bacteria does not have an insert. Presence of insert results into insertional inactivation of the -galactosidase and the colonies do not produce any colour, these are identified as recombinant colonies.

Selectable markers have been developed which differentiate recombinants from non- recombinants on the basis of their ability to produce colour in the presence of a chromogenic substrate. In this, a recombinant DNA is inserted within the coding sequence of an enzyme B-galactosidase. This results into an  insertional inactivation. The presence of chromogenic substrate gives blue coloured colonies if the plasmid in bacteria does not have an insert. Presence of insert results into insertional inactivation of a^-galactosidase enzyme and the colonies do not produce any colour, these are identified as recombinant colonies. 

Vectors, gene gun ( biolistic ) and microinjection are all viable techniques to insert foreign DNA into the host cell. So, the correct option is All of these.

There are various methods of making the host cell competent for transformation with Recombinant DNA technology.

• Biolistics (gene gun), Microinjection and Electroporation are direct methods of gene transfer.
  
• Agrobacterium tumefaciens is a gram-negative soil bacterium, which can cause crown gall tumors at wound sites of infected dicotyledonous plants.
• Agrobacterium tumefaciens acts as a vector for the transfer of genetic material hence it cannot be used for direct gene transfer.
• Hence Agrobacterium tumefaciens cannot be used for direct gene transfer.
• So, the correct answer is 'Agrobacterium tumefaciens'.

In gene therapy trials, the retrovirus is used as a vector to treat X-linked severe combined immunodeficiency (X-SCID) represent the most successful application of gene therapy. The retrovirus genetic material is in the form of RNA molecules, and their hosts genetic material is in the form of DNA. When a retrovirus infects a host cell, it will introduce its RNA together with some enzymes, namely reverse transcriptase and integrase, into the cell. This RNA molecule from the retrovirus must produce a DNA copy from its RNA molecule before it can be integrated into the genetic material of the host cell. The process of producing a DNA copy from an RNA molecule is termed reverse transcription.

In the gene gun method, microscopic pellets of gold or tungsten are coated with the transgene fragments and shot into the plant cells or tissues at high velocity. 

Electroporation or electropermeabilization, is a molecular biology technique in which an electrical field is applied to cells in order to increase the permeability of the cell membrane, allowing chemicals, drugs, or DNA to be introduced into the cell. In molecular biology, the process of electroporation is often used to transform bacteria, yeast, or plant protoplasts by introducing new coding DNA.

Pollen culture (microspore culture) is a technique in which haploid plants are obtained from isolated pollen grains. Pollen or microspore culture is an in vitro technique by which the pollen grains, preferably at the uninucleate stage, are squeezed out aseptically from the intact anther and then cultured on nutrient medium where the microscope, without producing male gametes, develop into haploid embryoids or callus tissues that give rise to haploid plantlets by embryogenesis or organogenesis. Polyethylene glycol (PEG)-mediated cell fusion is a simple and efficient technique used widely for the production of somatic cell hybrids and for nuclear transfer in mammalian cloning. Fusion can be performed between adherent and suspension cells or between adherent cells or suspension cells. Cytokinins are the key factor to induce the direct shoot regeneration. Azolla form a symbiotic relationship with the cyanobacterium, Anabaena azollae, which fixes atmospheric nitrogen, giving the plant access to the essential nutrient. Pectinase is an enzyme that breaks down pectin, a polysaccharide found in plant cell walls. Cellulases contribute to the enzymatic splitting of cellulose, a component of cell wall.

• Several chemicals have been used to induce protoplast fusion.
• Sodium nitrate (NaN03), polyethene glycol (PEG), Calcium ions (Ca2+), Polyvinyl alcohol etc. are the most commonly used protoplast fusion inducing agents which are commonly known as chemical fusogens. 
• Hence, PEG is used for Fusing of protoplasts.
•  So, the correct answer is 'Fusing of protoplasts'.

Meiotic recombination is the result of crossover and independent assortment. Crossover is the exchange of genetic material between non-sister chromatids of a homologous pair that generate new genetic combinations; option D is correct. Telomeres are the structures present at the ends of linear eukaryotic chromosomes and have many tandem copies of a short oligonucleotide sequence with guanine residues in one strand and cytosine in the complementary strand. The G-rich strand has a few hundred nucleotides long overhang at the 3’ end which serves as the primer for its elongation by telomerase enzyme and thereby maintaining chromosome length; option A is incorrect. The independent assortment of non-homologous chromosomes during meiosis results in independent segregation of nonhomologous chromosomes; option B is incorrect. Centromere directs the formation of the kinetochore on its surface, a special protein structure that attaches to the microtubules in the mitotic spindle; option C is incorrect. The correct answer is D.

Gene transfer technologies include techniques of electroporation, microinjection, macroinjection, biolistics or microprojectiles for DNA transfer (particle gun), liposome mediated gene transfer, calcium phosphate mediated DNA transfer, use of DAE-dextran to transfer gene, polycation-DMSO for DNA transfer, polyethylene glycol mediated transfection and gene transfer through peptides.