Among the following, the optically inactive compound is

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds is optically inactive, we need to understand the concept of chirality and optical activity.

### Step-by-Step Solution:

1. **Understanding Optical Activity**:
- Optical activity refers to the ability of a compound to rotate the plane of polarized light. A compound is said to be optically active if it can rotate polarized light, while it is optically inactive if it does not affect the plane of polarized light.

2. **Chirality**:
- A compound is chiral if it has a non-superimposable mirror image. This typically occurs when a carbon atom is bonded to four different groups (asymmetrical). If a compound has a plane of symmetry or is superimposable on its mirror image, it is considered achiral and thus optically inactive.

3. **Analyzing the Compounds**:
- **Compound 1**: A quaternary ammonium salt with four different groups attached to the nitrogen atom. This compound is chiral and optically active.
- **Compound 2**: A phosphorus compound with three different groups and a lone pair. The lone pair creates asymmetry, making this compound chiral and optically active.
- **Compound 3**: A carbon atom bonded to two identical groups and one different group. Since it has a plane of symmetry (the two identical groups), this compound is achiral and optically inactive.
- **Compound 4**: A carbon atom bonded to four different groups. This compound is chiral and optically active.

4. **Conclusion**:
- Among the compounds analyzed, **Compound 3** is the only optically inactive compound because it has a plane of symmetry and is not chiral.

### Final Answer:
The optically inactive compound is **Compound 3**.