Which one the following has largest number of isomers?

A

$$[Ru(NH_3)_4 Cl_2]^+$$

B

$$[Co(en)_2 Cl_2]^+$$

C

$$[Ir(PR_3)_2 H(CO)]^{2+}$$

D

$$[Co(NH_3)_5 Cl]^{2+}$$ (R = alkyl group, en = ethylenediamine)