**Core Concept**
The absorbance of a protein at 280 nm is primarily due to the presence of aromatic amino acids, such as tryptophan and tyrosine, which have a strong absorption peak at this wavelength. This property is used to estimate the concentration of proteins in solution.

**Why the Correct Answer is Right**
The correct answer is **C. Tryptophan** because it has a strong absorption peak at 280 nm due to its indole structure. Tryptophan contains a planar, conjugated ring system that absorbs light at this wavelength, resulting in an increase in absorbance. In contrast, alanine does not have any aromatic rings and therefore does not absorb light at 280 nm.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glycine is not an aromatic amino acid and does not absorb light at 280 nm. Its small size and absence of any chromophores make it an unlikely candidate for increasing absorbance.
* **Option B:** Glutamate is a non-aromatic amino acid and does not have any significant absorption at 280 nm. Its side chain is a carboxylic acid group, which does not contribute to absorption at this wavelength.
* **Option D:** Alanine (the original amino acid) is not an aromatic amino acid and does not absorb light at 280 nm. It is the reference amino acid used in this question.

**Clinical Pearl / High-Yield Fact**
Aromatic amino acids, such as tryptophan and tyrosine, are often used as markers for protein concentration in solution. This property is exploited in various biochemical assays, including protein quantification and spectroscopy.

**Correct Answer:** C. Tryptophan