Which of the following systems do not give correct discription of axial lengths and axial angles ?

Options

A. Hexagonal a=b`ne`c `alpha=beta=90^@, gamma=120^@`
B. Tetragonal `a=bnec alpha =beta=gamma=90^@`
C. Monoclinic `anebnec alpha=beta=gamma ne 90^@`
D. Cubic a=b=c `alpha=beta=gamma =90^@`

Answer

To determine which of the given crystal systems does not provide the correct description of axial lengths and angles, we will analyze each system based on their definitions.

### Step-by-Step Solution:

1. **Understanding Axial Lengths and Angles**:
- Axial lengths are represented as \( a, b, c \).
- Axial angles are represented as \( \alpha, \beta, \gamma \).

2. **Analyzing Each Crystal System**:
- **Hexagonal System**:
- Given: \( a = b \neq c \) and \( \alpha = \beta = 90^\circ, \gamma = 120^\circ \).
- This is correct as per the definition of the hexagonal system.

- **Tetragonal System**:
- Given: \( a = b \neq c \) and \( \alpha = \beta = \gamma = 90^\circ \).
- This is also correct according to the tetragonal system description.

- **Monoclinic System**:
- Given: \( a \neq b \neq c \) and \( \alpha = \beta = \gamma \neq 90^\circ \).
- This is incorrect. In the monoclinic system, \( a \neq b \neq c \) is correct, but the angles should be \( \alpha = \beta \neq 90^\circ \) and \( \gamma \neq 90^\circ \) (not equal to each other).

- **Cubic System**:
- Given: \( a = b = c \) and \( \alpha = \beta = \gamma = 90^\circ \).
- This is correct as per the cubic system definition.

3. **Conclusion**:
- The monoclinic system does not provide the correct description of axial lengths and angles as it incorrectly states that \( \alpha, \beta, \gamma \) are equal.

### Final Answer:
The system that does not give the correct description of axial lengths and axial angles is **Monoclinic (Option C)**.

---