Compound X reacts with $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{MgBr}$, and aqueous acid work-up gives a secondary alcohol Y. Oxidation of Y gives pentan-3-one. X is

Y must be pentan-3-ol, $(\mathrm{C}{}_2\mathrm{H}{}_5){}_2\mathrm{CHOH}$. A secondary alcohol arises from an aldehyde plus Grignard reagent, so the aldehyde must supply $\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{CH}$. Hence X is $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CHO}$; methanal would give a primary alcohol, ethanal would give butan-2-ol and butanal would give hexan-3-ol.