Which of the following is gem-dihalide?
\[A.{\text{ }}C{H_3}CHB{r_{2{\text{ }}}}{\text{ }} \]
\[B.{\text{ }}BrC{H_2}C{H_2}Br{\text{ }}\]
\[C.{\text{ }}C{H_3}CHBrC{H_2}Br{\text{ }}\]
\[D.{\text{ }}C{H_3}CHBrC{H_2}C{H_2}C{H_2}Br\]

Hint: The meaning of geminal-dihalides can be understood from the name itself. Germinal means twins and dihalides mean two halogen atoms. Dihalide is the di-halogen derivative of a halogen-containing organic compound. Thus a gem-dihalide or geminal-dihalide means a compound having two same halogen atoms present on the same carbon. You can draw the structures of the given options to guess the correct answer.

Complete answer:
Gem-dihalides are organic compounds that have the same halogen atom attached to the same carbon. They are also called alkylidene halides. To guess which among the above options is a gem-dihalide we need to write their structures individually. So, let us draw the drawing of all and see which among them is the correct answer.

When we take a look at the above structures we observe that except option (A) in all the other options the halogen atom ‘Br’ is the same but it is attached to two different carbon atoms. In option (B) and (C) it is attached to adjacent carbons that is carbon number $1{\text{ }}and{\text{ }}2$and in option (D) it is attached to carbon numbers $1{\text{ }}and{\text{ }}4$. And thus we can say that only option (A) is the gem-dihalide.
We see that in options (B) and (C) the Br group is attached to the adjacent carbons; such chemical compounds where the two same halogens are attached to adjacent carbons are called the vicinal-dihalides. These are formed when there is a reaction between a halogen and alkene.

Therefore the correct option is \[A.{\text{ }}C{H_3}CHB{r_{2{\text{ }}}}{\text{ }}\]

Note:
Geminal dihalides are prepared by the addition reaction of vinyl halide. When vinyl halide undergoes an additional reaction with hydrogen chloride then the formation of geminal dihalide takes place. Geminal di-halides and vicinal-dihalides are important concepts in organic chemistry.