It's B.
[Co (NH3)5cl]2 is optically inactive....

Explanation:-
It has an axis of symmetry that passses through Cl-Co-N, and it has also 4 planes of symmetry.
So, can't rotate the plane....

U can understand this by drawing it's octahedral shape,.......

Introduction

Optical activity is the ability of a substance to rotate the plane of polarized light. A compound is considered optically active if it contains chiral centers, which are carbon atoms bonded to four different groups. In this analysis, we will examine the given compounds and determine which one is not optically active.

Analysis

[Co (en)2Cl2]
- This compound contains two en (ethylenediamine) ligands, which are chelating agents and form five-membered rings with the central cobalt ion.
- Both en ligands have two different groups attached to the chiral carbon atom, making this compound optically active.

[Co (NH3)5Cl]2
- This compound consists of five NH3 (ammine) ligands and one Cl ligand coordinated to the central cobalt ion.
- None of the ligands in this compound have chiral centers, so it is not optically active.

[Co (NH3)4Cl2]
- This compound contains four NH3 (ammine) ligands and two Cl ligands coordinated to the central cobalt ion.
- Similar to the previous compound, none of the ligands in this compound have chiral centers, making it optically inactive.

[Cr (ox)3]-3
- This compound features three ox (oxalate) ligands coordinated to the central chromium ion.
- The ox ligands do not have chiral centers, so this compound is not optically active.

Conclusion

Based on the analysis, the compound that is not optically active is [Co (NH3)5Cl]2. This is because it does not contain any chiral centers in its ligands. The other three compounds have ligands with chiral centers, making them optically active.