Correct answer: A
Explanation
Sumatra is the geographically closest island to Great Nicobar, separated by the Great Channel (also known as the Six Degree Channel). The distance between the southernmost point of Great Nicobar and the northern tip of Sumatra (Aceh province, Indonesia) is approximately 145–150 km.
In contrast, Sri Lanka is located about 1,200 km to the west of the Nicobar Islands. Java is located to the southeast of Sumatra, and Borneo is situated much further east in the Malay Archipelago, making both significantly more distant from Great Nicobar than Sumatra.