**Core Concept**
Neutral amino acids are a class of amino acids that do not have a net charge at physiological pH. They are essential for various cellular functions, including protein synthesis and neurotransmitter production. Examples of neutral amino acids include alanine, valine, leucine, isoleucine, proline, glycine, serine, threonine, cysteine, and methionine.

**Why the Correct Answer is Right**
Neutral amino acids are characterized by the absence of ionizable groups, which means they do not have a pKa value close to physiological pH. As a result, they do not ionize and remain electrically neutral. This property allows them to cross cell membranes more easily and participate in various biochemical reactions. The absence of a net charge also enables them to interact with other molecules without being affected by electrostatic forces.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a particular characteristic of neutral amino acids.
**Option B:** This option is incorrect because it may refer to acidic or basic amino acids, which have a net charge at physiological pH.
**Option C:** This option is incorrect because it is too broad and includes all types of amino acids, not just neutral ones.

**Clinical Pearl / High-Yield Fact**
Neutral amino acids can be used as a substrate for amino acid transport across cell membranes. This property is exploited in certain clinical conditions, such as in the treatment of amino acid disorders or in the management of patients undergoing dialysis.

**Correct Answer: D. Neutral amino acids are a class of amino acids that do not have a net charge at physiological pH.**