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.
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