**Core Concept**
Zwitterions are a class of molecules that exhibit both acidic and basic properties, resulting in a net neutral charge. This unique property is due to the presence of both acidic and basic functional groups within the same molecule, leading to the formation of a dipolar ion. Zwitterions are commonly found in amino acids, peptides, and certain pharmaceutical compounds.

**Why the Correct Answer is Right**
The correct answer is related to the fact that zwitterions have a net neutral charge due to the equal number of positive and negative charges. In the context of amino acids, this occurs when the amino group (-NH2) donates a proton (H+), forming a positively charged ammonium group (-NH3+), while the carboxyl group (-COOH) accepts a proton, forming a negatively charged carboxylate group (-COO-). This balance of charges results in a net neutral charge for the molecule.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the charge characteristics of zwitterions.
**Option B:** This option is incorrect because it implies that zwitterions have a net positive or negative charge, which is not true.
**Option C:** This option is incorrect because it suggests that zwitterions are always charged, which is not the case; they can have a net neutral charge.

**Clinical Pearl / High-Yield Fact**
Zwitterions are an important class of compounds in pharmacology, as many medications exhibit zwitterionic properties. Understanding the charge characteristics of zwitterions is crucial for predicting their behavior in biological systems and optimizing their therapeutic effects.

**Correct Answer:** D.