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.