Correct Answer:
C. US
The United States has the highest number of airports in the world. This is attributed to its vast geographical area, highly developed aviation industry, extensive general aviation sector, and a large number of private airfields in addition to major commercial airports. The sheer scale of its economy and the historical development of air travel have led to an unparalleled aviation infrastructure. While countries like China, Brazil, and Russia have significant numbers of airports due to their size, they do not surpass the comprehensive network found in the US.