Assertion : In formaldehyde all the four atoms lie in one plane.
Reason : Carbonyl carbon forms a `pi`-bond with oxygen by overlapping of p -orbitals.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To analyze the assertion and reason provided in the question, we will break down the concepts step by step.

### Step 1: Understanding the Assertion
The assertion states that "In formaldehyde, all four atoms lie in one plane."

**Explanation**:
Formaldehyde (H₂C=O) consists of a carbon atom (C) double-bonded to an oxygen atom (O) and single-bonded to two hydrogen atoms (H). The carbon atom is at the center, with the two hydrogen atoms and the oxygen atom arranged around it.

### Step 2: Determining the Hybridization
To understand the geometry of formaldehyde, we need to determine the hybridization of the carbon atom.

**Explanation**:
The carbon atom in formaldehyde has three regions of electron density: one double bond with oxygen (which consists of one sigma and one pi bond) and two single bonds with hydrogen. The hybridization can be determined as follows:
- Number of sigma bonds = 3 (2 from H and 1 from O)
- Number of lone pairs on carbon = 0

Using the formula for hybridization:
\[ \text{Hybridization} = \text{Number of sigma bonds} + \text{Number of lone pairs} \]
\[ \text{Hybridization} = 3 + 0 = 3 \]
This indicates that the carbon is sp² hybridized.

### Step 3: Geometry of the Molecule
With sp² hybridization, the geometry of formaldehyde can be predicted.

**Explanation**:
In sp² hybridization, the molecular geometry is trigonal planar. This means that all atoms connected to the carbon atom (the two hydrogens and the oxygen) will lie in the same plane. Therefore, the assertion is true.

### Step 4: Analyzing the Reason
The reason states that "Carbonyl carbon forms a pi-bond with oxygen by overlapping of p-orbitals."

**Explanation**:
In formaldehyde, the carbon atom forms a double bond with the oxygen atom. This double bond consists of one sigma bond (formed by the overlap of sp² hybridized orbitals from carbon and an sp² hybridized orbital from oxygen) and one pi bond (formed by the side-to-side overlap of the unhybridized p-orbitals from carbon and oxygen). This pi bond contributes to the planar structure of the molecule.

### Step 5: Conclusion
Both the assertion and the reason are true, and the reason correctly explains the assertion.

**Final Answer**:
Both the assertion and the reason are correct, and the reason explains the assertion.