Solution

ADA or Adenosine deaminase is an enzyme produced by all cells in the body, however it is majorly produced by the Lymphocyte cells that undergo maturation in the Thymus gland and other lymph nodes. These cells defend the body against pathogen. This enzyme is released by lymphocytes to convert deoxyadenosine to deoxyinosine. Deoxyadenosine is released during the break down of DNA and is harmful to the lymphocytes and is hence converted to harmless deoxyinosine. So, the correct answer is 'Lymphocytes.'