Reagent which can detect the presence of `NH_(3)` is:

Options

A. Fehling's solution
B. Nessler's reagent
C. Benedict's solution
D. Lucas reagent

Explanation

To determine which reagent can detect the presence of ammonia (NH₃), we need to analyze the options provided:

1. **Felling solution**: This reagent is primarily used to detect the presence of aldehydes (CHO group). Since it is not used for ammonia detection, this option is incorrect.

2. **Benedict solution**: This solution is utilized for detecting reducing sugars, such as glucose and fructose. It does not react with ammonia, making this option incorrect as well.

3. **Lukas reagent**: This reagent is used to test for alcohols, specifically to differentiate between primary, secondary, and tertiary alcohols. It does not indicate the presence of ammonia, so this option is also incorrect.

4. **Nessler's reagent**: This reagent is specifically designed to detect ammonia (NH₃) and ammonium ions (NH₄⁺). When ammonia is present, Nessler's reagent produces a yellow to brown color, confirming the presence of ammonia.

Based on the analysis of the options, the correct answer is:

**Nessler's reagent (K₂HgI₄)** is the reagent that can detect the presence of NH₃.

### Step-by-Step Solution:
1. **Identify the reagents**: List the four options given in the question.
2. **Analyze each reagent**:
- **Felling solution**: Used for aldehydes, not ammonia.
- **Benedict solution**: Used for reducing sugars, not ammonia.
- **Lukas reagent**: Used for alcohols, not ammonia.
- **Nessler's reagent**: Specifically used for detecting ammonia.
3. **Determine the correct reagent**: Based on the analysis, Nessler's reagent is the only one that detects ammonia.
4. **State the formula**: The formula for Nessler's reagent is K₂HgI₄.