Which amino acid from following contains $-\mathrm{CH}_3$ as side chain ?

An amino acid's structure consists of a central alpha-carbon bonded to an amino group ($-\mathrm{NH}_2$), a carboxyl group ($-\mathrm{COOH}$), a hydrogen atom ($-\mathrm{H}$), and a side chain (R-group) that defines the amino acid's identity.

Analysis of the R-groups:

Leucine (A): $-$–$\mathrm{CH}_2\mathrm{CH}(\mathrm{CH}_3)_2$
Alanine (B): $-$–$\mathrm{CH}_3$
Serine (C): $-$–$\mathrm{CH}_2\mathrm{OH}$
Valine (D): $-$–$\mathrm{CH}(\mathrm{CH}_3)_2$

The methyl group ($-\mathrm{CH}_3$) appears as the R-group in alanine.