Two molecules of an aldehyde or ketone containing α- hydrogen atom, combine itself in the presence of dilute alkali to form β-hydroxyaldehyde or β-hydroxyketone. 

This reaction is called aldol condensation after the name of the product which contains both aldehyde (al) and a hydroxy (ol) group.

Similar condensation of two molecules of ketones ( α- hydrogen) is also considered as aldol condensation although the product in this case contains a keto and hydroxy group rather than an aldehyde group.

Which of the following compounds will not undergo aldol condensation?

Methanal

Out of the four given compounds only methanal does not have α-hydrogen. So, methanal will not undergo aldol condensation.