The correct answer is (a) Borneo. Borneo is the third largest island in the world, after Greenland and New Guinea. It is divided into three countries: Indonesia, Malaysia, and Brunei. The Indonesian part of Borneo is called Kalimantan. The Malaysian part is called Sarawak and Sabah. Brunei is a small country that is located on the northern coast of Borneo.
Celebes, New Guinea, and Timor are all islands that are located in the East Indies. However, they are not divided into three countries. Celebes is an island that is located in Indonesia. New Guinea is an island that is divided between Indonesia and Papua New Guinea. Timor is an island that is divided between Indonesia and East Timor.
Here is a map of the East Indies:
As you can see, Borneo is the only island in the East Indies that is divided into three countries.