Why D? And why not the others?

Correct answer: D. Ethiopia

Ethiopia's red, gold and green tricolour is the wellspring of the Pan-African colour scheme, because Ethiopia was one of the only African nations never colonised by a European power (aside from a brief Italian occupation from 1936 to 1941), making its flag a ready-made symbol of Black sovereignty during the 20th-century wave of independence. When Ghana became the first sub-Saharan colony to gain independence in 1957, it deliberately chose red, gold and green for its own flag as a tribute to Ethiopia, and dozens of newly independent nations followed suit in the following decades. Ghana is the natural trap since it is the country most associated with popularising the colours, but it was the follower, not the source. Liberia's flag instead echoes the American Stars and Stripes with red-and-white stripes and a single star, and Nigeria's flag uses only green and white, with no red or yellow at all.

Source: Britannica: flag of Ethiopia; Britannica: flag of Ghana