Which of the following compounds will not undergo aldol...

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

Acetaldehyde ($\text{CH}_3\text{-CHO}$) has three $\alpha$-hydrogen atoms.

Acetone ($\text{CH}_3\text{-CO-CH}_3$) has six $\alpha$-hydrogen atoms.

Propionaldehyde ($\text{CH}_3\text{-CH}_2\text{-CHO}$) has two $\alpha$-hydrogen atoms.

Benzaldehyde ($\text{C}_6\text{H}_5\text{-CHO}$) does not have any $\alpha$-hydrogen atom.

Therefore, benzaldehyde will not undergo aldol condensation.

Answer: $\text{C}_6\text{H}_5\text{-CHO}$ (Benzaldehyde)