If it is an aliphatic diol, i.e straight chain then you cannot have geometric isomers.
I think the point here is that you can get erythro/threo isomers.
For example, if you take Z-but-2-ene and treat it with OsO
4 you will not get the same isomer as when you treat E-but-2-ene with OsO
4.
Similarly, treatment of Z-but-2-ene with OsO
4 will give different compounds than treatment with mCPBA.
sahar58 - look up these mechanisms of dihydroxylation and attempt the question. Showing an attempt is required - see forum rules.