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

Explanation

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

### Step 1: Understanding the Assertion
The assertion states that formaldehyde is a planar molecule. To determine if this is true, we need to look at the structure of formaldehyde.

**Formaldehyde Structure**:
- The molecular formula of formaldehyde is CH₂O.
- It consists of one carbon atom (C) double-bonded to an oxygen atom (O) and single-bonded to two hydrogen atoms (H).

### Step 2: Drawing the Structure
The structure can be represented as follows:
```
H
|
H—C=O
```
In this structure, the carbon atom is at the center, with the two hydrogen atoms and the oxygen atom around it.

### Step 3: Analyzing the Geometry
To understand the geometry, we need to consider the hybridization of the carbon atom:
- The carbon atom in formaldehyde is bonded to two hydrogen atoms and one oxygen atom via a double bond.
- This means that the carbon atom uses three hybrid orbitals to form these bonds.

### Step 4: Determining Hybridization
The hybridization of the carbon atom can be determined as follows:
- The carbon atom is bonded to three regions of electron density (two single bonds with hydrogen and one double bond with oxygen).
- This corresponds to **sp² hybridization** because:
- One s orbital and two p orbitals mix to form three sp² hybrid orbitals.
- The remaining p orbital is used to form the π bond in the double bond with oxygen.

### Step 5: Conclusion on Planarity
Since the carbon atom is sp² hybridized, the molecular geometry is trigonal planar. Thus, all atoms in formaldehyde lie in the same plane.

### Final Answer
- **Assertion (A)**: True, formaldehyde is a planar molecule.
- **Reason (R)**: True, it contains an sp² hybridized carbon atom.

Both the assertion and the reason are correct.