The alkyne shown gives two isomeric ketones on hydration with $\mathrm{HgSO}{}_4$/$\mathrm{H}{}_2\mathrm{SO}{}_4$. Which of the following alkynes gives only one ketone under the same conditions?

The compound shown is pent-2-yne, whose two alkyne carbons are different, so pentan-2-one and pentan-3-one both form. Only a symmetrical internal alkyne gives one ketone: hex-3-yne gives only hexan-3-one. The other three alkynes are unsymmetrical and each gives two ketones.