Why A? And why not the others?

Correct answer: A. Greenland

Greenland covers roughly 2,166,000 square kilometers, making it the largest body of land on Earth that reference works still classify as an island. New Guinea is the genuine runner-up, the second-largest true island, at under half Greenland's area. Borneo ranks third among islands, smaller still. Australia is the tempting trap here: it is comparable in scale and often pictured as 'just a very large island,' but geographers and standard references classify it as a continent in its own right rather than an island, precisely because of its size and the fact that it sits on its own continental plate rather than being a fragment of a larger landmass. That classification line, not raw square kilometers, is what keeps Australia off the island leaderboard entirely.

Source: Encyclopaedia Britannica: 'The Largest Islands in the World' (britannica.com/list/the-largest-islands-in-the-world)