Assertion : In methanal, all the four atoms are in the same plane.
Reason : The carbon atom in methanal is `sp^2` hybridized.

Answer

To solve the question, we need to analyze the assertion and reason provided:

**Assertion**: In methanal, all the four atoms are in the same plane.
**Reason**: The carbon atom in methanal is `sp^2` hybridized.

### Step-by-Step Solution:

1. **Understanding Methanal Structure**:
- Methanal (commonly known as formaldehyde) has the chemical formula CH₂O. It consists of one carbon atom (C), two hydrogen atoms (H), and one oxygen atom (O).

2. **Determining Hybridization of Carbon**:
- The carbon atom in methanal is bonded to two hydrogen atoms and one oxygen atom through a double bond (C=O).
- To find the hybridization, we count the number of sigma bonds and lone pairs around the carbon atom:
- The carbon forms:
- 2 sigma bonds with the two hydrogen atoms (C-H)
- 1 sigma bond with the oxygen atom (C-O)
- The double bond with oxygen consists of 1 sigma bond and 1 pi bond.
- Total sigma bonds = 2 (C-H) + 1 (C-O) = 3 sigma bonds.
- There are no lone pairs on the carbon atom.

3. **Calculating Hybridization**:
- The hybridization can be determined using the formula:
\[ \text{Number of sigma bonds + Number of lone pairs} = 3 + 0 = 3 \]
- Since the sum is equal to 3, the hybridization of the carbon atom is `sp^2`.

4. **Geometric Arrangement**:
- The `sp^2` hybridization indicates that the geometry around the carbon atom is trigonal planar.
- In a trigonal planar arrangement, all the atoms bonded to the carbon atom (the two hydrogen atoms and the oxygen atom) lie in the same plane.

5. **Conclusion**:
- Since the carbon atom in methanal is `sp^2` hybridized, and this hybridization leads to a trigonal planar geometry, we can conclude that all four atoms (C, H, H, O) are indeed in the same plane.
- Therefore, both the assertion and the reason are correct, and the reason correctly explains the assertion.

### Final Answer:
Both the assertion and reason are correct, and the reason is the correct explanation of the assertion.

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