Why A? And why not the others?

Correct answer: A. Cyprus

Cyprus's 1960 independence flag shows a copper-coloured silhouette of the island above two olive branches, on a plain white field — one of the very few national flags ever to depict the outline of its own territory. The copper tone is a nod to the island's name: the ancient Greek word for the island, tied to its copper deposits, passed into Latin as 'cuprum' and gave English the word 'copper', since Cyprus supplied much of the ancient Mediterranean's copper ore. Bermuda's flag instead carries a red field with a coat of arms showing a lion and a shipwreck, not a map. Bangladesh's flag is a plain red disc set on green, with no territorial outline at all, and Eswatini's flag bears a ceremonial shield and spears rather than a map.

Source: Britannica: flag of Cyprus; Oxford English Dictionary: etymology of 'copper'