Why B? And why not the others?

Correct answer: B. The Bay of Bengal

The Bay of Bengal, the northeastern arm of the Indian Ocean bordered by India, Bangladesh, Myanmar, and Sri Lanka, covers roughly 2.2 million square kilometers, making it the largest bay on Earth by area. The Gulf of Mexico is the natural rival to consider, since gulfs and bays are both large curved indentations of a coastline and it is itself one of the largest bodies of its kind in the world, but at under 1.6 million square kilometers it still falls well short. Hudson Bay is an enormous inland sea covering much of northeastern Canada, genuinely vast at over a million square kilometers, yet still smaller than the Bay of Bengal. The Great Australian Bight is a huge open indentation along Australia's southern coastline and is often pictured on maps as similarly massive, but it covers a far smaller area than any of the other options here.

Source: Encyclopaedia Britannica: 'Bay of Bengal' (britannica.com/place/Bay-of-Bengal)