Counting the number of steriosomers
 

The following question comes from a sample test and the answer is given as choice D. I have no idea what the letter "D" in the middle of the molecule means. I have heard of D and L configurations but those letters are usually written at the beginning of the molecule.

Could this be a typo? Thanks.

D is deuterium, a heavy isotope of hydrogen, it has relative atomic mass = 2.

It is important in this question, because if it was H instead of a D, there would only be three steroisomers, SS, RR and RS (which =SR). You'll see this if you draw them out.

Since you have one OH and one OD, you actually have 4 stereoisomers, because now that the deuterium is there, the SR isomer is not the same as the RS.