How is D a meso compound? It isn't. But it is achiral bc it has a mirror plane of symmetry. In addition there are NO chiral centers in D (look again and convince yourself of that). A quick inspection of answer C shows that the configuration at both chiral centers are R, so thats out. Fact is there are NO meso compounds listed in the answer choices. But the question isn't asking for meso, it's asking for achiral. All achiral means is that the molecule is superimposable on its mirror image.
Meso is a special kind of achiral compound. As in any achiral molecule there will be an internal mirror plane. In addition meso requires 2 or more IDENTICALLY substituted stereocenters having opposite R and S configurations that cancel out any optical activity.
It may benefit you to keep this in mind: All chiral molecules will have at least 1 chiral center ( not really but for the DAT they will.).
Any molecule that lacks chiral centers is achiral ( and will have a mirror plane of symmetry). But there is ONE case where you will have chiral centers and an achiral molecule....you guessed it meso!!!
