Aspartame has two chiral carbon atoms from its amino acid residues

Steps:

• Aspartame is the dipeptide L-aspartyl-L-phenylalanine methyl ester.
• A chiral carbon has four different substituents attached.
• The alpha carbon of the aspartic acid residue bonds to H, amino group (peptide-linked), carboxyl (peptide-linked), and -CH2COOH side chain—making it chiral.
• The alpha carbon of the phenylalanine residue bonds to H, amino group (from peptide), carboxyl (esterified), and -CH2C6H5 side chain—making it chiral.
• Side chain carbons are -CH2- groups with two identical H atoms, so achiral; no other carbons qualify.

Why B is correct:

• Aspartame's structure features exactly two tetrahedral carbons (alpha carbons of Asp and Phe) each with four distinct substituents, per the definition of chirality.

Why the others are wrong:

• A: Ignores the chiral alpha carbon in the phenylalanine residue.
• C: Includes nonexistent chiral centers, like the achiral beta carbons.
• D: Overcounts severely, as no additional asymmetric carbons exist.

Final answer: B