The answer is B. But why? What makes it so different than the others? I know that it attacks from the back (anti addition). But can't it do that for all and result in a meso compound?
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A meso compound is a molecule with multiple stereocenters that is superimposable on its mirror image. Meso compounds are easily identified by their internal plane of symmetry.
Using this knowledge, and given that $\ce{Br2}$ undergoes anti addition to alkenes, there are only really two answers that could possibly be meso compounds, B and D. Let us consider the products:
Notice how only B has an internal plane of symmetry, and is therefore a meso compound.
