Why B? And why not the others?

Correct answer: B. 206

A typical adult skeleton has 206 bones, but here is the twist: you were born with around 300. The figure of three hundred is tempting precisely because it is true of newborns, whose skeletons include many separate, partly cartilaginous pieces. As we grow, dozens of these fuse together, so the count falls without a single bone being lost: the skull plates knit into a solid braincase, and five sacral vertebrae merge into one sacrum at the base of the spine. The counts of 270 and 186 are plausible near-misses, with 270 sitting in the range sometimes quoted for infants and 186 simply undershooting the adult total.

Source: Cleveland Clinic: How many bones does a baby have; Nemours KidsHealth: Your Bones