Basic crystal systems known are:

Options

A. 7
B. 8
C. 6
D. 4

Explanation

To answer the question about the basic crystal systems, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Total Number of Crystal Systems**:
- There are a total of 7 basic crystal systems in crystallography.

2. **List the Crystal Systems**:
- The 7 crystal systems are:
1. **Cubic**: All sides are equal and all angles are 90 degrees.
2. **Tetragonal**: Two sides are equal, and all angles are 90 degrees.
3. **Orthorhombic**: All sides are unequal, but all angles are 90 degrees.
4. **Monoclinic**: All sides are unequal, with one angle not equal to 90 degrees.
5. **Triclinic**: All sides are unequal, and all angles are unequal (none are 90 degrees).
6. **Hexagonal**: Two sides are equal, with angles of 120 degrees between them and 90 degrees with the third side.
7. **Rhombohedral (or Trigonal)**: All sides are equal, but angles are not 90 degrees.

3. **Summarize the Information**:
- In summary, the 7 basic crystal systems are Cubic, Tetragonal, Orthorhombic, Monoclinic, Triclinic, Hexagonal, and Rhombohedral.

### Final Answer:
The basic crystal systems known are:
1. Cubic
2. Tetragonal
3. Orthorhombic
4. Monoclinic
5. Triclinic
6. Hexagonal
7. Rhombohedral

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