$B.$ Treatment of alkenes with bromine $\left( Br _2\right)$ gives vicinal dibromides $(1,2-$ dibromides). Notes: The bromines add to opposite faces of the double bond ("anti addition"). Sometimes the solvent is mentioned in this reaction - a common solvent is carbon tetrachloride $\left( CCl _4\right)$.

$C.$ Dihydroxylation of alkenes with cold, dilute $KMnO _4$ to give vicinal diols.

Description: Treatment of alkenes with cold, dilute basic $KMnO _4$ leads to $1,2$-diols (vicinal diols).

$D.$ Ammonical $Cu _2 Cl _2$ identification test for terminal alkynes and not for alkene and alkane. In this reaction, an terminal alkyne reacts with basic cupric chloride and on subsequent oxidation in air gives diyne. For example, ethyne couples with itself to produce but-$1,3$-yne (red brown ppt). therefore on above explanation, we say that Option $D$ is a correct answer.