Key idea: reactivity of aldehydes toward nucleophilic addition

Nucleophilic addition to a carbonyl group depends mainly on:

• Electrophilicity of the carbonyl carbon
• Steric hindrance around the carbonyl carbon

For aldehydes, reactivity increases when:

• Electron-donating effect on the carbonyl carbon is less
• Steric hindrance is less

General order among aldehydes

The aldehyde with the least alkyl donation and least steric bulk is most reactive.

Hence, the general order is: $\text{HCHO} > \text{aliphatic aldehydes} > \text{aromatic aldehydes}$

This is because:

• In formaldehyde, HCHO, there is no alkyl group donating electron density.
• It also has minimum steric hindrance.
• Aromatic aldehydes are less reactive because the benzene ring conjugates with the carbonyl group, reducing the positive character on the carbonyl carbon.

Conclusion

Therefore, the most reactive aldehyde toward nucleophilic addition is formaldehyde (methanal).

Comparison with stored answer

The options in the prompt are not visible, so I cannot map the compound directly to option A/B/C/D from the given image-less text.

• If option B = formaldehyde (HCHO / methanal), then the stored answer is correct.
• Otherwise, the stored answer would be incorrect.

Based on chemistry, the correct compound is: $\boxed{\text{HCHO (methanal)}}$