In coronary artery by-pass surgery the blood vessel to be used a graft has to be

Coronary artery bypass grafting (CABG) needs a conduit to carry blood past a blocked coronary segment. Surgeons use several vessels, and all three options are used in practice, so D is the defensible answer.

• A (Great saphenous vein from the leg): the most commonly used conduit, easy to harvest and useful for multiple grafts. Its long-term patency is weaker because it is prone to atherosclerosis.
• B (Radial artery from the forearm): an arterial graft with better long-term patency than vein. It is used increasingly, but needs adequate ulnar collateral flow, checked by Allen's test.
• C (Internal mammary artery from the chest): the best graft for the left anterior descending artery, with excellent long-term patency, usually left attached at its origin.

Core concept: the graft is a substitute conduit, and the choice depends on patient anatomy, vessel size and target artery, not on one fixed vessel. This links to Human Physiology and medical science in General Science. The Mains angle is the role of affordable cardiac care and access to advanced surgery in India.

Trap: students pick A or C because they remember one standard graft, and the word "has to be" tempts a single answer. The question asks what can be used, and all three are.