Which of the following exhibits geometrical isomerism?

Geometrical isomerism occurs in alkenes when each carbon atom of the double bond is attached to two different groups. Let us examine the structures of the given compounds.

For 1, 2-dibromopropene, the structure is $CH_3-C(Br)=CHBr$. The carbon at position 2 is attached to a methyl group and a bromine atom. The carbon at position 1 is attached to a hydrogen atom and a bromine atom. Since both carbons have two different groups attached, this compound exhibits geometrical isomerism (cis and trans forms).

For 2-methylbut-2-ene, the structure is $(CH_3)_2C=CHCH_3$. The carbon at position 2 is attached to two identical methyl groups, so it does not exhibit geometrical isomerism.

For hex-1-ene, the structure is $CH_2=CHCH_2CH_2CH_2CH_3$. The terminal carbon is attached to two identical hydrogen atoms, so it does not exhibit geometrical isomerism.

For 1, 1-diphenylethene, the structure is $(C_6H_5)_2C=CH_2$. The carbon at position 1 is attached to two identical phenyl groups, so it does not exhibit geometrical isomerism.

Answer: 1, 2-Dibromopropene