Which of the following is not a planar molecule?

Options

A. (a)`H_(2)C =C=CH_(2)`
B. (b)`H_(2)C=C=C=CH_(2)`
C. (c )`H_(2)C=C=O`
D. (d)`NC-HC=CH-CN`

Explanation

To determine which of the given molecules is not planar, we need to analyze the hybridization of the atoms in each molecule. A molecule is generally considered planar if all its atoms lie in the same geometric plane, which is typically the case for molecules with sp² or sp hybridization.

### Step-by-Step Solution:

1. **Identify the Hybridization of Atoms**:
- For each molecule, identify the hybridization of the central atoms.
- **sp² hybridization** occurs when there are three sigma bonds and no lone pairs, while **sp hybridization** occurs when there are two sigma bonds and one or more lone pairs.

2. **Analyze Each Option**:
- **Option A**: Contains carbon atoms with double bonds. The central carbon is sp hybridized, while the others are sp². This configuration suggests that the molecule is planar.
- **Option B**: Similar to option A, with carbon atoms involved in double bonds. The hybridization indicates planarity.
- **Option C**: Contains an oxygen atom with lone pairs. The presence of lone pairs can influence the geometry, but the hybridization suggests that it remains planar.
- **Option D**: Contains a carbon atom with a triple bond to nitrogen. The carbon is sp hybridized, and the nitrogen is also sp hybridized. However, the presence of multiple bonds and lone pairs can lead to a non-planar structure.

3. **Determine the Non-Planar Molecule**:
- After analyzing the hybridization and bonding, it is evident that **Option A** is the correct answer. The presence of multiple double bonds in a molecule can lead to a non-planar structure due to steric hindrance and the arrangement of the p orbitals.

### Conclusion:
The molecule that is not planar is **Option A**.