Accepting the definition that an acid is a proton donor, the acid in the following reaction
`NH_(3)+H_(2)O rarr NH_(4)^(+)+OH^(-)` is

Options

A. `NH_(3)`
B. `H^(+)`
C. `NH_(4)^(+)`
D. `H_(2)O`

Answer

To determine the acid in the reaction \( \text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4^+ + \text{OH}^- \) based on the definition that an acid is a proton donor, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reactants**: The reactants in the reaction are ammonia (\( \text{NH}_3 \)) and water (\( \text{H}_2\text{O} \)).

2. **Analyze the Reaction**: In the reaction, ammonia (\( \text{NH}_3 \)) is converted to ammonium ion (\( \text{NH}_4^+ \)), and water (\( \text{H}_2\text{O} \)) is converted to hydroxide ion (\( \text{OH}^- \)).

3. **Determine Proton Transfer**:
- Ammonia (\( \text{NH}_3 \)) is accepting a proton (\( \text{H}^+ \)) to form ammonium ion (\( \text{NH}_4^+ \)).
- Water (\( \text{H}_2\text{O} \)) is losing a proton (\( \text{H}^+ \)) to form hydroxide ion (\( \text{OH}^- \)).

4. **Identify the Acid**: According to the Bronsted-Lowry theory, an acid is defined as a substance that donates a proton. Since water (\( \text{H}_2\text{O} \)) is losing a proton, it acts as the acid in this reaction.

5. **Conclusion**: Therefore, the acid in the reaction is water (\( \text{H}_2\text{O} \)).

### Final Answer:
The acid in the reaction is \( \text{H}_2\text{O} \).