$$\begin{gathered} \left(\mathrm{I}\right) \mathrm{HCHO} \left(\mathrm{II}\right) {\mathrm{CH}}_3\mathrm{CHO} \\ \left(\mathrm{III}\right) {\mathrm{CCl}}_3\mathrm{CHO} \left(\mathrm{IV}\right) {\mathrm{CH}}_3{\mathrm{COCH}}_3 \\ \left(\mathrm{V}\right) {\mathrm{CCl}}_3{\mathrm{COCH}}_3 \left(\mathrm{VI}\right) {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CHO} \end{gathered}$$

Which of the above compounds undergo aldol condensation? 

For aldol condensation reaction a compound must have alpha-hydrogen atom. Since, compound I, II & VI have no alpha-hydrogen atom present so they are not able to go under aldol condensation while others have alpha-hydrogen atom. 

Hence, the correct answer is option (B) Only II, IV and V.