Correct Option(1) Retrovirus

Explanation:

Usually, a retrovirus is used as a vector for introducing a DNA fragment in human cells. They have used as a vector in gene therapy to introduce the desired gene so as to replace the functioning of a defected gene e.g. Severe Combined Immune.

Deficiency (SCID) is caused due to a defect in the gene for the enzyme adenosine deaminase.

In gene therapy against it, lymphocytes are extracted from the bone marrow of the patient. These are grown in a culture outside the body. A functional ADA rDNA, using a retroviral vector, is then introduced into these lymphocytes. These are reinjected into the patient’s bone marrow.

λ-phage allows cloning of DNA fragments upto 23 kb lengths. Ti-plasmid is usually used for plants. pBR-322 is an artificial cloning vector, usually used for bacteria.