The crystal system having the highest and the lowest symmetries respectively , are

Options

A. cubic and rhombohedral
B. cubic and triclinic
C. rhombohedral and monolinic
D. cubic and monoclinic

Answer

To solve the question regarding the crystal systems with the highest and lowest symmetries, we need to analyze the seven crystal systems based on their symmetry properties.

### Step-by-Step Solution:

1. **Identify the Seven Crystal Systems**:
The seven crystal systems are:
- Cubic
- Tetragonal
- Orthorhombic
- Rhombohedral
- Hexagonal
- Monoclinic
- Triclinic

2. **Understand Symmetry in Crystal Systems**:
- **Cubic**: All three axes are equal (a = b = c) and all angles are 90° (α = β = γ = 90°). This system has the highest symmetry.
- **Tetragonal**: Two axes are equal (a = b ≠ c) and all angles are 90° (α = β = γ = 90°).
- **Orthorhombic**: All three axes are unequal (a ≠ b ≠ c) but all angles are 90° (α = β = γ = 90°).
- **Rhombohedral**: All three axes are equal (a = b = c) but angles are not 90° (α = β = γ ≠ 90°).
- **Hexagonal**: Two axes are equal (a = b ≠ c) and angles are 90° and 120° (α = β = 90°, γ = 120°).
- **Monoclinic**: All axes are unequal (a ≠ b ≠ c) and two angles are 90° while one is not (α = γ = 90°, β ≠ 90°).
- **Triclinic**: All axes are unequal (a ≠ b ≠ c) and all angles are unequal (α ≠ β ≠ γ).

3. **Determine the Highest Symmetry**:
The cubic system has the highest symmetry because all axes are equal and all angles are equal to 90°.

4. **Determine the Lowest Symmetry**:
The triclinic system has the lowest symmetry because all axes are unequal and all angles are also unequal.

5. **Conclusion**:
Therefore, the crystal system with the highest symmetry is **Cubic**, and the one with the lowest symmetry is **Triclinic**.

### Final Answer:
The crystal system having the highest and the lowest symmetries respectively are:
- Highest Symmetry: **Cubic**
- Lowest Symmetry: **Triclinic**