The correct structure of dipeptide, Gly-Ala (glycyl alanine) is:

The correct answer is: H₂N- CH₂- CO-NH-CH(CH₃)-COOH

Step-by-Step Explanation:

1. Understanding Dipeptides:
A dipeptide is formed by the condensation reaction between two amino acids, where the carboxyl group ($-\mathrm{COOH}$) of one amino acid reacts with the amino group ($-{\mathrm{NH}}_2$) of another amino acid. This reaction eliminates a water molecule ($H_{2}O$) and forms a peptide bond ($-\mathrm{CO}-\mathrm{NH}-$).

2. Identifying the Constituent Amino Acids:
The dipeptide is Gly-Ala (glycyl alanine). By convention, the amino acid written first (Gly, Glycine) provides the acyl group (containing the carbonyl $-\mathrm{CO}-$ component of the peptide bond), and the second amino acid (Ala, Alanine) provides the nitrogen atom ($-\mathrm{NH}-$) of the peptide bond.
- Glycine (Gly): The simplest amino acid, with the structure $H_{2}N-{\mathrm{CH}}_2-\mathrm{COOH}$.
- Alanine (Ala): An amino acid with a methyl side chain, having the structure $H_{2}N-\mathrm{CH}\left({\mathrm{CH}}_3\right)-\mathrm{COOH}$.

3. Forming the Peptide Bond:
To form Gly-Ala:
- The $-\mathrm{COOH}$ group of Glycine ($H_{2}N-{\mathrm{CH}}_2-\mathrm{COOH}$) reacts with the $-{\mathrm{NH}}_2$ group of Alanine ($H_{2}N-\mathrm{CH}\left({\mathrm{CH}}_3\right)-\mathrm{COOH}$).
- Removal of a water molecule ($-\mathrm{OH}$ from Glycine's carboxyl group and $-H$ from Alanine's amino group) yields the peptide link:
$-\mathrm{CO}-\mathrm{NH}-$

4. Assembling the Final Structure:
Combining the remaining residues in order from the N-terminus (left) to the C-terminus (right):
- N-terminus residue (Glycine portion): $H_{2}N-{\mathrm{CH}}_2-\mathrm{CO}-$
- Peptide linkage: $-\mathrm{NH}-$
- C-terminus residue (Alanine portion): $-\mathrm{CH}\left({\mathrm{CH}}_3\right)-\mathrm{COOH}$

Putting it all together, we get:
H₂N- CH₂- CO-NH-CH(CH₃)-COOH