Four enatiomeric products can be formed, two pairs of mirror image enantiomers, and these two pairs will be diastereoisomers of each other. The reason for this selectivity of diastereo is that the transition states are different...the rate is different...and the products have different properties since they are not mirror image compounds.
For this problem you need to know the mechanism of the reaction...that's pretty much all there is to it.
Once you do you can know the enatiomeric pair B (picked due to diastereoselectivity) which is a diastereo of B*, the enantiomeric pair which is not formed.