Final answer:
The mainland of Southeast Asia is classified as a peninsula, being a landmass that is largely surrounded by water but connected to the mainland of Asia. The correct answer is option B.
Step-by-step explanation:
The mainland of Southeast Asia is a landform that can be categorised as a peninsula. This mainland region borders China and India and contains extensive river systems, acting as a significant geographic feature of Southeast Asia. The mainland is connected to the rest of the continent and extends southwards towards the equator, fitting the general definition of a peninsula, which is a piece of land surrounded by water on most of its border while being connected to a mainland.