Within the list shown below, the correct pair of structures of alanine in pH ranges 2-4 and 9-11 is
I. `H_(3)N^(+)CH(CH_(3))CO_(2)H`
II. `H_(2)NCH(CH_(3))CO_(2)^(-)`
III. `H_(3)N^(+)CH(CH_(3))CO_(2)^(-)`
IV. `H_(2)NCH(CH_(3))CO_(2)H`

Options

A. I,II
B. I,III
C. II, III
D. III,IV

Explanation

To solve the question regarding the correct pair of structures of alanine in the specified pH ranges, we need to analyze the structures provided and understand how the protonation and deprotonation of alanine occurs at different pH levels.

### Step-by-Step Solution:

1. **Understanding the pH Ranges**:
- **pH 2-4**: This is an acidic range. In this range, the carboxylic acid group (-COOH) of alanine will be protonated, and the amino group (-NH2) will be protonated as well, forming a positively charged ammonium ion (-NH3+).
- **pH 9-11**: This is a basic range. In this range, the carboxylic acid group will be deprotonated, resulting in a negatively charged carboxylate ion (-COO-), while the amino group remains in its neutral form (-NH2).

2. **Analyzing the Structures**:
- **Structure I**: `H3N+CH(CH3)CO2H` - This structure has a protonated amino group and a protonated carboxylic acid, suitable for pH 2-4.
- **Structure II**: `H2NCH(CH3)CO2-` - This structure has a neutral amino group and a deprotonated carboxylic acid, suitable for pH 9-11.
- **Structure III**: `H3N+CH(CH3)CO2-` - This structure has a protonated amino group and a deprotonated carboxylic acid, which does not fit either pH range correctly.
- **Structure IV**: `H2NCH(CH3)CO2H` - This structure has a neutral amino group and a protonated carboxylic acid, which is not suitable for pH 9-11.

3. **Identifying the Correct Pairs**:
- For pH 2-4, Structure I is suitable.
- For pH 9-11, Structure II is suitable.
- Therefore, the correct pair of structures of alanine in the specified pH ranges is **I and II**.

### Final Answer:
The correct pair of structures of alanine in pH ranges 2-4 and 9-11 is **I and II**.