Greenland is a big island. the continent of africa is much larger than greenland. but on some maps greenland and africa appear to be almost the same size. why?

Green land and Africa appear to be of the same size because the Mercator projection creates distortions in size as we move away from the equator. The distortion is severe near the poles, this inability to compare size on Mercator projection is called The Greenland problem. Although Greenland is fourteen times bigger than Africa it appears to be smaller than it. The cropping technique used in Mercator projection causes these distortions.

you didn't list the options. however, a few communist countries today are: the people's republic of china, north korea, and cuba.