Which of the following compound is most reactive towards nucleophilic addition reaction?

Formaldehyde

The correct answer is Option (4) → Formaldehyde

The most reactive compound towards nucleophilic addition reaction is:

 Formaldehyde 

• Reactivity order: Formaldehyde > Other aldehydes > Ketones
• Formaldehyde has no alkyl groups, so:
• Least steric hindrance
• No +I effect to reduce the electrophilicity of the carbonyl carbon

Hence, it is attacked most easily by nucleophiles.