Which of the following alkyl halides gives a mixture of alkenes on...

Dehydrohalogenation of alkyl halides involves the removal of a hydrogen atom from a $\beta$-carbon and a halogen atom from the $\alpha$-carbon to form an alkene.

For n-propyl bromide (CH_3-CH_2-CH_2-Br$), there is only one \beta$-carbon, yielding only propene.

For isopropyl bromide (CH_3-CH(Br)-CH_3$), the two \beta$-carbons are equivalent, yielding only propene.

For secondary butyl bromide (CH_3-CH_2-CH(Br)-CH_3$), there are two non-equivalent \beta$-carbons. Elimination of hydrogen from these different $\beta$-carbons yields a mixture of 1-butene and 2-butene (which further exists as cis and trans isomers).

For tertiary butyl bromide ($(CH_3)_3C-Br$), all three $\beta$-carbons are equivalent, yielding only 2-methylpropene.

Thus, secondary butyl bromide gives a mixture of alkenes.

Answer: Secondary butyl bromide