Why C? And why not the others?

Correct answer: C. Seven

A giraffe has just seven cervical vertebrae — exactly the same number as a human, a mouse, and almost every other mammal. Instead of adding bones, evolution stretched each one: a single giraffe neck vertebra can be about 25 centimetres (10 inches) long, linked by ball-and-socket-style joints that keep the neck surprisingly flexible. Guesses like twelve, fourteen or twenty-four feel right because a neck that long intuitively should have extra segments — and in birds it genuinely does, since swans carry more than 20 neck vertebrae. But the mammalian body plan is remarkably locked at seven, a constraint so strong that even whales, which barely have visible necks at all, still keep seven compressed cervical vertebrae. The giraffe is simply the most dramatic demonstration of how far nature can stretch a fixed blueprint rather than redesign it.

Source: Cleveland Zoological Society: giraffes have seven cervical vertebrae, the same as humans