Identify what each test is used for

We need the test which cannot be used for identifying amino acids.

Check each option

A. Ninhydrin test

• Ninhydrin reacts with amino acids containing free $-NH_2$ groups.
• It gives a characteristic purple/violet color.
• Hence, it is used for identification of amino acids.

B. Barfoed test

• Barfoed test is used to distinguish monosaccharides from disaccharides.
• It is a test for carbohydrates, not amino acids.
• Hence, it cannot be used for identifying amino acids.

C. Xanthoproteic test

• This test detects aromatic amino acids such as tyrosine, tryptophan, and phenylalanine by nitration, giving a yellow/orange color.
• So it can be used for certain amino acids.

D. Biuret test

• Biuret test is mainly for proteins/peptides because it detects peptide bonds.
• Free amino acids generally do not respond positively.
• However, in standard biomolecule test questions, the clearly unrelated test for amino acids is Barfoed test, which is a carbohydrate test.

Conclusion

The test that cannot be used for identifying amino acids is: $\boxed{\text{B. Barfoed test}}$

Comparison with stored answer

Stored correct answer: B

My derived answer: B

Therefore, they agree.