**Core Concept**
The question tests the understanding of amino acid structure and their contribution to protein absorbance at 280 nm, which is primarily due to the presence of **aromatic amino acids** like tryptophan, tyrosine, and phenylalanine.

**Why the Correct Answer is Right**
Substitution of alanine with an amino acid that has a higher absorbance at 280 nm would increase the protein's absorbance. Tryptophan has a high absorbance at 280 nm due to its **indole ring**. When tryptophan is substituted in place of alanine, it increases the absorbance of the protein at 280 nm.

**Why Each Wrong Option is Incorrect**
**Option A:**
**Option B:**
**Option C:**
Since the actual options are missing, let's assume the typical incorrect options would be other amino acids without significant absorbance at 280 nm, such as glycine, valine, or leucine.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **tryptophan** is the primary contributor to protein absorbance at 280 nm due to its unique **indole structure**. This is crucial in protein quantification methods.

**Correct Answer:** D. Tryptophan