Write the compound clearly

Nitropentammine chromium $(III)$ chloride is: $[Cr(NH_3)_5(NO_2)]Cl_2$

Here:

• $Cr$ is in oxidation state $+3$
• $NH_3$ is a neutral ligand
• $NO_2^-$ is an ambidentate ligand
• $2Cl^-$ are counter ions

Identify the special ligand

The ligand $NO_2^-$ is ambidentate, meaning it can coordinate through two different atoms:

• through N atom: nitro form
• through O atom: nitrito form

So the complex can exist as: $[Cr(NH_3)_5(NO_2)]Cl_2$ and $[Cr(NH_3)_5(ONO)]Cl_2$

Type of isomerism due to ambidentate ligand

When a ligand attaches through different donor atoms, the isomerism is called linkage isomerism.

Check other options

• Optical isomerism: not possible here for this octahedral complex of type $[MA_5B]$
• Ionization isomerism: would require exchange between coordinated ion and counter ion; not applicable here
• Polymerisation isomerism: not relevant

Conclusion

The isomerism present is linkage isomerism.

Therefore, the correct option is: $\boxed{B}$