• Chemical Structure of Aspartame: Aspartame is an artificial, non-nutritive dipeptide sweetener chemically designated as L-aspartyl-L-phenylalanine methyl ester ($N$-L-$\alpha$-aspartyl-L-phenylalanine 1-methyl ester).
• Stereospecificity of Taste: Only the L,L-enantiomer binds productively to human sweet-taste receptor heterodimers (T1R2/T1R3), providing approximately 180–200 times the sweetness of sucrose.
• Inactivity of Other Isomers: The stereoisomeric forms (such as D-aspartyl-L-phenylalanine methyl ester or D-aspartyl-D-phenylalanine methyl ester) taste intensely bitter or completely tasteless.

Correct option: (4) L-aspartyl-L-phenylalanine methyl ester