Hydroxamic acid test is employed to detect-A.KetonesB.AldehydesC.EstersD.Amides

The hydroxamic acid test is a chemical test used primarily to detect the presence of **aldehydes** and **ketones**.

### Explanation:

1. **Hydroxamic Acids**:
- Hydroxamic acids are formed when hydroxylamine reacts with carbonyl compounds (aldehydes or ketones). The general reaction involves treating a carbonyl compound with hydroxylamine to produce a hydroxamic acid.

2. **Mechanism**:
- The reaction can be summarized as follows:
\[
\text{RCHO (aldehyde) or R}_2\text{C=O (ketone) + NH}_2\text{OH (hydroxylamine) } \rightarrow \text{RCO}(\text{OH})\text{NHOH (hydroxamic acid)}
\]
- This hydroxamic acid can then be treated with iron (III) chloride (FeCl₃), which results in the formation of a colored complex, usually purple or red, indicating the presence of carbonyl compounds.

3. **Options**:
- **A. Ketones**: Detectable by the hydroxamic acid test.
- **B. Aldehydes**: Detectable by the hydroxamic acid test.
- **C. Esters**: Generally do not react in the hydroxamic acid test.
- **D. Amides**: Also do not give a positive reaction in this test.

### Conclusion:
The hydroxamic acid test is specifically used to detect **Aldehydes (B)** and **Ketones (A)**. However, since the question asks for what the test is employed to detect and gives only one answer option, the most common answer is **B. Aldehydes**. If both options were available, both B and A could be correct.

Thus, if you must choose one answer from the given options, **B. Aldehydes** would be the most appropriate.