Why A? And why not the others?

Correct answer: A. Tooth enamel

Tooth enamel is the hardest substance the body makes, and the reason is its extreme mineral content: roughly 96 percent of enamel is tightly packed hydroxyapatite crystal, more mineral than any other tissue we produce. The trade-off is that mature enamel contains no living cells, so once it is worn or dissolved by acid it cannot grow back. The femur's dense compact bone is the tempting runner-up because the femur is famously the body's longest and strongest bone, yet bone is only about two-thirds mineral and softer than enamel. The kneecap is ordinary bone despite its knock-resistant reputation, and cartilage is deliberately soft and springy, cushioning joints rather than resisting scratches.

Source: NIH news release: Complexity of human tooth enamel revealed at atomic level (NIDCR)