Transplantation of organs is necessary when:

The correct answer is Option (1) → Organs are failing to function

When organs such as the heart, eye, liver or kidney fail to function satisfactorily, transplantation is often the only remedy that lets the patient live a normal life. Before the transplant, tissue matching and blood group matching are done, because the body’s cell-mediated immune response can reject a graft it treats as ‘non-self’.

Option (1) Organs are failing to function — Correct. Transplantation is needed when an organ fails to function satisfactorily.

Option (2) Antibodies are produced in excess — Incorrect. The book does not link transplantation to excess antibody production. Antibodies are made by B-lymphocytes in response to pathogens.

Option (3) The immune system is weakened — Incorrect. A weak immune system is not a reason for transplantation. After a transplant, the patient in fact takes immuno-suppressants all his/her life to keep the graft from being rejected.

Option (4) B-lymphocytes are not functioning properly — Incorrect. B-lymphocytes produce antibodies; a fault in them is not given as a reason for an organ transplant.

NCERT Extract:
Acquired Immunity – “… Very often, when some human organs like heart, eye, liver, kidney fail to function satisfactorily, transplantation is the only remedy to enable the patient to live a normal life. Then a search begins – to find a suitable donor. … Tissue matching, blood group matching are essential before undertaking any graft/transplant and even after this the patient has to take immuno– suppresants all his/her life.”