Substitution of which one of the following amino acids in place of alanine would increase the absorbance of protein at 280 nm

Amino acids do not absorb visible light and thus are colourless. However, tyrosine, phenylalanine, and especially tryptophan absorb high-wavelength (250-290 nm) ultraviolet light. Because it absorbs ultraviolet light about ten times more efficiently than either phenylalanine or tyrosine, tryptophan makes the major contribution to the ability of most proteins of absorbing light in the region of 280 nm.Ref: Harper 28th Ed. Page-18, Harper 27th Ed. Page no: 18