Which of the following is commonly used as a vector for introducing a DNA fragment in human lymphocytes?

The correct option is D Retrovirus
Retrovirus
Retroviruses are a class of viruses possessing single stranded RNA as their genetic material.
Lymphocytes from the patient’s blood are isolated. A functional ADA cDNA is introduced into them using a retroviral vector. Genetically engineered lymphocytes are then infused back into the patient’s blood. The modified lymphocytes start producing functional ADA.

Ti plasmid
Ti plasmid is a tumour inducing plasmid of Agrobacterium tumefaciens that are modified to be used as a vector in plants which is no more pathogenic to the plants but is still able to use the mechanisms to deliver genes of our interest into a variety of plants.

λ phage
λ phage vector is the genetic material present in the λ phage virus that infects bacteria such as E. coli.

pBR322
pBR322 is a plasmid. It is a widely used E. coli cloning vector. It is used to create recombinant DNA and obtain multiple copies of the alien gene.

Final answer:
(a) Retrovirus