Step 1: Recall the reducing nature of diborane.
Diborane \((\mathrm{B_2H_6})\) is a strong reducing agent commonly used to reduce carboxylic acids to primary alcohols. Step 2: Analyze each functional group. (A) Nitro group: Reduced by agents like Sn/HCl or Fe/HCl, not diborane. (B) Halo group: Generally not reduced by diborane. (C) Ester group: Requires stronger reducing agents like \(\mathrm{LiAlH_4}\). (D) Carboxyl group: Correct — diborane selectively reduces carboxylic acids to alcohols. Step 3: Conclusion.
Diborane specifically reduces the carboxyl group.