Indonesia, Japan, and the Philippines differ from other parts of Southeast Asia because they are

a) more mountainous.

b) island nations.

c) landlocked.

d) less populous.

They're definitely not landlocked: for example, Japan is placed completely on islands.

Further, Indonesia is the most populous country in Southeast Asia - so they can't be "less populous"

Also, if they were more mountaneous than the others, then Myanmar would also be listed with them.

The best answer is: they're all island nations. This is also true: they are all island nations.