The two geometric isomers of but-2-ene are treated separately with (i) Br2 in CCl4 (anti addition through a bromonium ion) or (ii) cold dilute…

Anti addition to the trans alkene and syn addition to the cis alkene both place the two new groups so that the two stereocentres have opposite configurations $(R,S)$ with a mirror plane in the eclipsed conformation, giving the meso compound. The opposite pairings (anti to cis, syn to trans) give $(R,R)/(S,S)$, formed as a racemic mixture of chiral enantiomers. I confirmed this by building the 3D geometry of each addition and assigning CIP labels: trans plus $\mathrm{Br_2}$ gives $(R,S)$, cis plus $\mathrm{KMnO_4}$ gives $(R,S)$, while cis plus $\mathrm{Br_2}$ and trans plus $\mathrm{OsO_4}$ give $(S,S)$. Hence A is wrong (cis alkene with $\mathrm{Br_2}$ gives the racemate), B is right (2,3-dibromobutane, meso), C is right (butane-2,3-diol, meso), and D is wrong (trans alkene with $\mathrm{OsO_4}$ gives racemic diol). Answer: B, C.