Recall the general structure of an amino acid

Amino acids are organic compounds that contain both:

• an amino group: $-NH_2$
• a carboxylic acid group: $-COOH$

Their general structure is: $H_2N-CH(R)-COOH$ where $R$ is the side chain.

Check each option

• A: $-COOH$ group
  This is present in amino acids. ✅

• B: $-NH_2$ group
  This is also present in amino acids. ✅

• C: $-CH_3$ group
  This is not present in all amino acids. Only some amino acids may have a methyl group in the side chain. ❌

• D: both (a) and (b)
  Since amino acids contain both $-COOH$ and $-NH_2$ groups, this is correct. ✅

Final answer

The correct option is: $\boxed{\text{D: both (a) and (b)}}$

Comparison with stored answer

Stored correct answer: D
Derived answer: D
Therefore, they agree.