Among the following alcohols, which is the weakest acid in aqueous solution?

In water the acidity of alcohols falls as the alkyl group gets bulkier, because the large alkoxide ion is poorly solvated and the alkyl groups push electrons onto oxygen. The order is $\mathrm{CH}{}_3\mathrm{OH}>\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}>(\mathrm{CH}{}_3){}_2\mathrm{CHOH}>(\mathrm{CH}{}_3){}_3\mathrm{COH}$. So the tertiary alcohol is weakest.