Which of the following reagents is used to protect the amino group of the N-terminal residue in solution-phase peptide synthesis?

A) lithium diisopropyl amide
B) benzyl chloroformate
C) phenyl isothiocyanate
D) dicyclohexylcarbodiimide
E) trifluoroacetic acid

Benzyl chloroformate (also known as Z-Cl) is commonly used to protect the N-terminal amino group in solution-phase peptide synthesis. It reacts with the amino group to form a carbamate, which can be removed later by treatment with a mild base. The other reagents listed are not used for N-terminal protection, but rather for other steps in the peptide synthesis process.