JEE Main · 2004 · 24 Apr 2004 · Offline · Chemical Bonding and Molecular Structure

Look for where each molecule holds its $s{p}^2$ carbon, the trigonal carbonyl carbon. Acetone, acetamide and acetic acid each contain a $\mathrm{C}=\mathrm{O}$ group and so each has an $s{p}^2$ carbon. Acetonitrile, $\mathrm{CH}{}_3\mathrm{CN}$, has only an $s{p}^3$ methyl carbon and an $sp$ carbon in the $\mathrm{C}\equiv \mathrm{N}$ triple bond, with no $s{p}^2$ carbon anywhere, so it is the one lacking $s{p}^2$ hybridisation.