Why A? And why not the others?

Correct answer: A. The Antarctic Circumpolar Current

The Antarctic Circumpolar Current is the only current on the planet that flows all the way around the globe without ever being blocked by a continent, because at Antarctica's latitude there is an unbroken ring of open ocean; NOAA describes it as the dominant circulation feature of the Southern Ocean for exactly this reason. The Gulf Stream is a powerful and famous warm-water current, but it is confined to the North Atlantic, hemmed in by the Americas on one side and Europe and Africa on the other. The Kuroshio Current plays a similar role to the Gulf Stream but in the North Pacific, carrying warm water past Japan, yet it too is boxed in by surrounding landmasses. The Humboldt Current runs along the west coast of South America and strongly shapes the climate and fisheries there, but it is a regional coastal current rather than one that circles the entire Earth.

Source: NOAA Atlantic Oceanographic and Meteorological Laboratory: 'Antarctic Circumpolar Current' (aoml.noaa.gov/phod/altimetry/cvar/acc/index.php)