Assertion (A) Formaldehyde is a planar molecule.
Reason (R) It contains `sp^(2)` hybridised carbon atom.

Options

A. If both (A) and (R) are correct and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct.

Explanation

To solve the question regarding the assertion and reason related to formaldehyde, we will break it down step by step.

### Step 1: Understand the Structure of Formaldehyde
Formaldehyde has the molecular formula \( H_2CO \). Its structure can be represented as:
```
H
|
H--C==O
```
In this structure, the carbon atom is double-bonded to an oxygen atom and single-bonded to two hydrogen atoms.

**Hint:** Identify the molecular formula and draw the structure to visualize the bonding.

### Step 2: Determine the Hybridization of the Carbon Atom
The carbon atom in formaldehyde forms:
- 1 double bond with oxygen (which consists of 1 sigma bond and 1 pi bond)
- 2 single bonds with hydrogen atoms (2 sigma bonds)

Thus, the carbon atom forms a total of 3 sigma bonds.

**Hint:** Count the number of sigma bonds to determine the steric number.

### Step 3: Calculate the Steric Number
The steric number is calculated as follows:
\[ \text{Steric Number} = \text{Number of Sigma Bonds} + \text{Number of Lone Pairs} \]
In formaldehyde, there are 3 sigma bonds and no lone pairs on the carbon atom:
\[ \text{Steric Number} = 3 + 0 = 3 \]

**Hint:** Remember that lone pairs on the central atom also contribute to the steric number.

### Step 4: Determine the Hybridization Based on Steric Number
A steric number of 3 indicates that the hybridization of the carbon atom is \( sp^2 \). This hybridization leads to a trigonal planar geometry.

**Hint:** Use the steric number to identify the type of hybridization and the corresponding molecular geometry.

### Step 5: Conclude the Planarity of the Molecule
Since the carbon atom in formaldehyde is \( sp^2 \) hybridized, it adopts a trigonal planar shape, which means that all atoms bonded to the carbon atom lie in the same plane. Therefore, formaldehyde is indeed a planar molecule.

**Hint:** Connect the hybridization type to the molecular geometry to confirm the planarity.

### Final Conclusion
- **Assertion (A)**: Formaldehyde is a planar molecule. **(True)**
- **Reason (R)**: It contains \( sp^2 \) hybridized carbon atom. **(True)**
- The reason correctly explains the assertion.

Thus, the correct answer is that both A and R are correct, and R is the correct explanation of A.