The simplest amino acid is

The correct option is Glycine.

To understand why glycine is the simplest amino acid, let us look at the general structure of an amino acid:
Each amino acid consists of a central carbon atom (known as the alpha-carbon, C_α) bonded to four different groups:
1. An amino group (-NH₂)
2. A carboxyl group (-COOH)
3. A hydrogen atom (-H)
4. A variable side chain or R-group (-R)

The complexity and unique properties of each amino acid are determined entirely by this variable R-group (side chain).
In the case of glycine, the R-group is simply a single hydrogen atom (-H). Since hydrogen is the smallest and simplest possible atom, glycine has the smallest and simplest side chain of all the standard amino acids. Because its side chain contains only a hydrogen atom, glycine is also the only standard amino acid that is achiral (it does not have two distinct optical isomers because its central carbon is bonded to two identical hydrogen atoms).

For comparison, let's look at the side chains of the other options:
- Alanine: Has a methyl group (-CH₃) as its side chain, making it more complex than glycine.
- Asparagine: Has a carboxamide side chain (-CH₂CONH₂), which is significantly larger and polar.
- Tyrosine: Contains a large aromatic ring structure (-CH₂-C₆H₄-OH).

Therefore, because its side chain consists of just a single hydrogen atom, glycine is structurally the simplest amino acid.