which of these reactions is an addition reaction?\no h4c2 + br2 → h5c2br + hbr\no h4c2 + hbr → h5c2br\no…

Brief Explanations:

Addition reactions involve two or more molecules combining to form a single product. In the first reaction, there is also a by - product (HBr), so it is not an addition reaction. In the third reaction, it is a substitution reaction where Cl is replaced by OH. The second reaction shows ethene ($H_4C_2$) reacting with HBr to form a single product ethyl bromide ($H_5C_2Br$), which is characteristic of an addition reaction.

Answer:

$H_4C_2 + HBr\rightarrow H_5C_2Br$