Which of the following is commonly used as a vector for introducing a DNA fragment in human lymphocytes during gene therapy for curing ADA deficiency?

The correct option is A Retrovirus
Vectors are nucleotide sequences which are used as a vehicle to carry the gene of interest into the host organism. Plasmids, retrovirus, bacteriophages are some of the different types of vectors.

ADA or Adenosine deaminase deficiency causes a rare genetic disorder of the immune system known as Severe Combined Immunodeficiency. It is caused due to a deletion mutation in the gene that codes for ADA enzyme which is essential for the proper functioning of the immune system.
One of the treatment for ADA deficiency is gene therapy.
In this process a functional ADA gene is introduced into the lymphocytes followed by their infusion into the patient

In ADA gene therapy, lymphocytes from the patient’s body are grown in a culture medium outside the body. A functional ADA cDNA is introduced into the lymphocytes using a retroviral vector. The advantage of this is that retrovirus vectors are capable of transforming their ssRNA genome into a dsDNA molecule that stably integrates itself into the host cell’s genome. The genetically modified lymphocytes are returned to the patient’s blood.

Agrobacterium tumefaciens is a bacterial plant pathogen which carries the Ti plasmid. The T-DNA segment of the Ti plasmid integrates itself into the plant cell’s genome to induce tumour formation. This characteristic of the plasmid is used to transfer the gene of interest into the plant cell.

pBR322 is a genetically engineered plasmid vector.

Lambda (ƛ) phages are used for transferring genetic material to bacteria by infecting them.