**Core Concept**
The classification of amino acids into different categories is based on their side chain properties. Amino acids can be acidic, basic, nonpolar, or polar. Acidic amino acids have a side chain that can donate a proton (H+ ion), making them negatively charged.

**Why the Correct Answer is Right**
Aspartic acid (Asp or D) is an acidic amino acid because its side chain contains a carboxyl group (-COOH). When this group donates a proton, it becomes a carboxylate ion (-COO-), giving the molecule a negative charge. This property allows aspartic acid to participate in various biochemical reactions, such as protein synthesis and neurotransmission.

**Why Each Wrong Option is Incorrect**
**Option A:** Glutamic acid (Glu or E) is also an acidic amino acid, but it's not the correct answer here. While it is similar to aspartic acid, it has a slightly different side chain.
**Option B:** Serine (Ser or S) is a polar, uncharged amino acid. It does not have a carboxyl group in its side chain, so it cannot donate a proton.
**Option C:** Histidine (His or H) is a basic amino acid, not acidic. Its side chain contains an imidazole ring, which can accept a proton and become positively charged.

**Clinical Pearl / High-Yield Fact**
Remember that amino acids are the building blocks of proteins, and their properties determine the structure and function of proteins. Acidic amino acids like aspartic acid play crucial roles in protein synthesis, enzyme activity, and neurotransmission.

**Correct Answer: D. Aspartic acid.**