Base catalyzed aldol condensation occurs with

Aldol condensation is given by aldehydes and ketones that contain at least one $\alpha$-hydrogen atom.

In 2,2-dimethyl propanal, the $\alpha$-carbon is attached to three methyl groups and has no $\alpha$-hydrogen.

In benzaldehyde, the $\alpha$-carbon is part of the benzene ring and has no $\alpha$-hydrogen.

In propanal (CH_3-CH_2-CHO$), the \alpha$-carbon has two $\alpha$-hydrogen atoms. Therefore, it undergoes base-catalyzed aldol condensation.

Answer: Propanal