Sumatra is the geographically closest island to Great Nicobar. Great Nicobar is part of the Nicobar Islands of India, located north of Sumatra (Indonesia). Borneo is a much larger island further east in Southeast Asia. Java is another large island in Indonesia, but further west compared to Sumatra. Sri Lanka is a separate island nation south of India, significantly farther away than Sumatra. Therefore, the answer is (A) Sumatra.