a
only those compounds which have \(\alpha -H\) give aldol condensation

  \((a)\) \(\underset{(no\,\,\alpha -H)}{\mathop{\begin{matrix}
   \,\,\,\,\,\,\,\,\,\,\,O  \\
   \,\,\,\,\,\,\,\,\,\,\,||  \\
   C{{l}_{3}}\overset{\alpha }{\mathop{C}}\,CH  \\
\end{matrix}}}\,\)     \((b)\) \(\underset{(\alpha -H)}{\mathop{{{C}_{6}}{{H}_{5}}\overset{\alpha }{\mathop{C}}\,{{H}_{2}}CHO}}\,\)

\((c)\) \(\underset{(\alpha -H)}{\mathop{{{C}_{4}}{{H}_{9}}\overset{\alpha }{\mathop{C}}\,{{H}_{2}}CHO}}\,\)       \((d)\)  \(CH_3NO_2\)