**Core Concept**
The question requires understanding the properties of amino acids, specifically their polarity. Amino acids can be classified based on their side chain properties, which determine their polarity. In general, amino acids with polar side chains are more soluble in water.

**Why the Correct Answer is Right**
Polarity in amino acids is determined by the presence of electronegative atoms such as oxygen, nitrogen, or sulfur in their side chains. Glycine (C) has a side chain with a hydrogen atom, making it nonpolar. Valine (B) and Isoleucine (D) have nonpolar aliphatic side chains. Alanine (A) has a polar side chain due to the presence of a carboxyl group (-COOH), but not as polar as some other amino acids. However, among the given options, Glycine (C) has the least polarity due to its nonpolar side chain. The correct sequence from least polar to most polar is: Glycine (C), Valine (B), Isoleucine (D), and then Alanine (A).

**Why Each Wrong Option is Incorrect**
**Option A:** Alanine is not the least polar option among the given choices.
**Option B:** Valine and Isoleucine are nonpolar, but not the least polar.
**Option D:** Isoleucine is nonpolar, but not the least polar compared to Glycine.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that polarity in amino acids affects their solubility, which can impact their transport across cell membranes. Amino acids with nonpolar side chains tend to be more lipophilic and can be transported via facilitated diffusion, whereas polar amino acids may require active transport.

**Correct Answer: C. Glycine**