Which of the following statements is incorrect ?

Options

A. `FeCl_(3)` is used in the detection of phenols.
B. Fehling solution is used in the detection of glucose.
C. Tollens' reagent is used in the detection of unsaturation.
D. `NaHSO_(3)` is used in the detection of carbonyl compounds.

Explanation

To determine which statement is incorrect, let's analyze each statement one by one:

1. **Statement 1**: "FeCl3 is used in the detection of phenols."
- This statement is **correct**. Ferric chloride (FeCl3) reacts with phenols to form a violet complex, which indicates the presence of phenolic compounds.

2. **Statement 2**: "Felling solution is used in the detection of glucose."
- This statement is also **correct**. Felling's solution contains cupric ions (Cu²⁺) that are reduced to cuprous oxide (Cu₂O) when glucose is present. The formation of a red precipitate indicates the presence of glucose.

3. **Statement 3**: "Tollens reagent is used in the detection of unsaturation."
- This statement is **incorrect**. Tollens' reagent is specifically used to detect aldehydes, not unsaturation. It differentiates aldehydes from ketones and does not react with unsaturated compounds in the same way.

4. **Statement 4**: "Sodium bisulphite is used in the detection of carbonyl compounds."
- This statement is **correct**. Sodium bisulfite reacts with carbonyl compounds (aldehydes and ketones) to form bisulfite addition compounds, which can be used to identify the presence of carbonyl groups.

Based on the analysis, the incorrect statement is **Statement 3**: "Tollens reagent is used in the detection of unsaturation."

### Summary of the Solution:
- Statement 1: Correct (FeCl3 detects phenols)
- Statement 2: Correct (Felling's solution detects glucose)
- Statement 3: Incorrect (Tollens reagent detects aldehydes, not unsaturation)
- Statement 4: Correct (Sodium bisulfite detects carbonyl compounds)

### Final Answer:
The incorrect statement is **Statement 3**.