1. Identify the type of complexes and possible isomerism

We compare the number of isomers possible for each coordination compound.

Possible types of isomerism in coordination compounds:

• geometrical isomerism
• optical isomerism
• linkage/ionization/hydrate isomerism only if applicable

Here, only stereoisomerism matters.

2. Option A: $[Ru(NH_3)_4Cl_2]^+$

This is of the type $[MA_4B_2]$, generally octahedral.

Such complexes show geometrical isomerism:

• cis
• trans

So total number of isomers = 2.

3. Option B: $[Co(en)_2Cl_2]^+$

This is an octahedral complex of type $[M(AA)_2B_2]$, where $AA =$ bidentate ligand (en).

Such complexes show:

• cis form
• trans form

Now check optical activity:

• cis-$[M(en)_2Cl_2]$ is optically active, giving a pair of enantiomers: $d$ and $l$ forms.
• trans-$[M(en)_2Cl_2]$ is optically inactive.

Therefore total isomers:

• 2 optical isomers from cis
• 1 trans isomer

Total = 3 isomers.

4. Option C: $[Ir(PR_3)_2H(CO)]^{2+}$

This is a 4-coordinate complex with ligands:

• $2PR_3$
• $H$
• $CO$

For $d^8$ Ir(I)-type complexes, such complexes are typically square planar.

This is of the type $[MA_2BC]$ in square planar geometry.

Square planar $[MA_2BC]$ shows geometrical isomerism:

• A ligands adjacent to each other
• A ligands opposite to each other

So total isomers = 2.

No optical isomerism here.

5. Option D: $[Co(NH_3)_5Cl]^{2+}$

This is of the type $[MA_5B]$, octahedral.

Such a complex has only one arrangement.

So total isomers = 1.

6. Compare all options

• A: $2$
• B: $3$
• C: $2$
• D: $1$

Hence the complex with the largest number of isomers is $[Co(en)_2Cl_2]^+$

So the correct option is B.

7. Verification with stored answer

Stored correct answer: B

Our derived answer: B

They agree.