The compound with only primary hydrogen atoms is/are:

Options

A. Hexamethylcyclopropane
B. Neohexane
C. Tetramethylbutane
D. Hexamethylbenzene

Explanation

To determine which compounds have only primary hydrogen atoms, we need to analyze the structures of the given compounds one by one.

### Step-by-Step Solution:

1. **Hexamethylcyclopropane**:
- Structure: Cyclopropane consists of three carbon atoms arranged in a triangle. If we add six methyl groups (CH₃) to each carbon in the cyclopropane, we get the structure:
```
CH3
/ \
CH3 CH3
\ /
C
/ \
CH3 CH3
```
- Analysis: Each carbon in the cyclopropane is bonded to only one other carbon (the two adjacent carbons), making all the carbons primary (1°). Each methyl group contributes three primary hydrogen atoms. Therefore, the total number of primary hydrogen atoms is:
- 6 methyl groups × 3 H = 18 primary hydrogens.
- Conclusion: This compound has only primary hydrogen atoms.

2. **Neohexane**:
- Structure: Neohexane has a branched structure:
```
CH3
|
CH3 - C - CH2 - CH3
|
CH3
```
- Analysis: In neohexane, the carbon in the center is a secondary carbon (2°) because it is bonded to two other carbons. The primary carbons (1°) are the terminal methyl groups. The total number of primary hydrogens is:
- 4 CH₃ groups × 3 H = 12 primary hydrogens.
- Conclusion: This compound has both primary and secondary hydrogen atoms.

3. **Tetramethylbutane**:
- Structure: The structure of tetramethylbutane is:
```
CH3
|
CH3 - C - CH3
|
CH3
```
- Analysis: All the carbons in tetramethylbutane are primary (1°) because they are each bonded to only one other carbon. The total number of primary hydrogens is:
- 4 CH₃ groups × 3 H = 12 primary hydrogens.
- Conclusion: This compound has only primary hydrogen atoms.

4. **Hexamethylbenzene**:
- Structure: The structure of hexamethylbenzene is:
```
CH3
/ | \
CH3 C CH3
\ | /
CH
/ | \
CH3 C CH3
\ | /
CH3
```
- Analysis: In hexamethylbenzene, the carbon atoms in the benzene ring are bonded to each other and have double bonds, which means they cannot be primary. Each carbon in the ring is bonded to two other carbons, making them secondary (2°). The total number of primary hydrogens is:
- 6 CH₃ groups × 3 H = 18 primary hydrogens.
- Conclusion: This compound has only primary hydrogen atoms.

### Final Conclusion:
The compounds that contain only primary hydrogen atoms are:
- Hexamethylcyclopropane (1)
- Tetramethylbutane (3)
- Hexamethylbenzene (4)

Thus, the answer is **1, 3, and 4**.