Asn (N-terminus): Contains a free primary amine ($$-NH_2$$) and a side-chain primary amide ($$-CONH_2$$).

Ser (C-terminus): Contains a side-chain hydroxyl group ($$-OH$$).

Acetic anhydride ($$Ac_2O$$) in excess acts as a powerful acetylating agent. In the presence of a base ($$Et_3N$$), it replaces the active hydrogen atoms of nucleophilic functional groups with acetyl groups ($$-COCH_3$$ or $$-Ac$$).

Under excess conditions, all accessible nucleophilic sites are acetylated:

N-terminal Amine: $$-NH_2 \xrightarrow{Ac_2O} -NHCOCH_3$$ (Forms an acetamide).

Serine Side-chain: $$-OH \xrightarrow{Ac_2O} -OCOCH_3$$ (Forms an ester).

Asparagine Side-chain: $$-CONH_2 \xrightarrow{Ac_2O} -CONH-COCH_3$$ (Primary amides are acetylated to $$N$$-acetyl amides when the reagent is in excess).

Option B is the correct product because it correctly shows the acetylation of all three reactive sites: the N-terminus, the serine hydroxyl, and the asparagine side-chain amide. The C-terminal carboxylic acid ($$-COOH$$) remains in its acidic form.