Why B? And why not the others?

Correct answer: B. The hyoid, in the front of the neck

The U-shaped hyoid, sitting at the base of the tongue, touches no other bone at all. Instead it hangs in a sling of muscles and ligaments tethered to the jaw, skull, and voice box, and that free-floating suspension is precisely what makes it useful: it gives the tongue and swallowing muscles a mobile anchor, essential for chewing, swallowing, and speech. The kneecap is a tempting pick because it famously floats inside the quadriceps tendon, yet its back surface still glides directly against the femur. The stapes is joined to its neighboring ossicle, the incus, and the tailbone connects to the sacrum above it, so both fail the no-contact test.

Source: Encyclopaedia Britannica: hyoid bone