An ester of formula $\mathrm{C}{}_4\mathrm{H}{}_8\mathrm{O}{}_2$ is treated with excess $\mathrm{CH}{}_3\mathrm{MgBr}$ and then $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The only organic product is propan-2-ol. The ester is

A formate ester with excess Grignard reagent gives a secondary alcohol from the acyl part: $\mathrm{HCOOR}+2\mathrm{CH}{}_3\mathrm{MgBr}$ gives $(\mathrm{CH}{}_3){}_2\mathrm{CH}-\mathrm{OH}$. For only propan-2-ol to form, the alkoxy part must also be isopropyl. So the ester is $\mathrm{HCOOCH}(\mathrm{CH}{}_3){}_2$. Propyl methanoate would also give propan-1-ol; ethyl ethanoate gives 2-methylpropan-2-ol and ethanol.