Why A? And why not the others?

Correct answer: A. The Turritopsis dohrnii jellyfish

When Turritopsis dohrnii, the 'immortal jellyfish,' is injured, starving or simply aging, it can collapse its adult swimming body into a blob of tissue and reorganize itself back into a polyp — the juvenile, seafloor-attached stage — through a process called transdifferentiation, in which fully mature cells switch into entirely new cell types. In principle it can repeat this cycle indefinitely, though in the wild most individuals are eventually eaten. The tardigrade is tempting because of its legendary indestructibility, but it survives extremes by shutting its metabolism down almost completely, not by growing young again. The axolotl can regrow lost limbs and even parts of its heart and brain, yet its body still ages forward like any other salamander's. The planarian flatworm regenerates a whole body from a fragment, but it never rewinds through its life stages the way this fingernail-sized jellyfish does.

Source: Natural History Museum (London): immortal jellyfish Turritopsis dohrnii — life cycle reversal via transdifferentiation