If a metal is bound to only one kind of donor group in coordination sphere, then it is called homoleptic complex.
Metals bounded to more than one kind of donor group is called heteroleptic complex.
[Co(NH3)6]Cl3 consists of only ammonia ligands in the coordination sphere and hence, it is homoleptic. The rest of the compounds consists of two kinds of ligands, either NH3 and CO3 and hence, they are heteroleptic.
Hence, option (a) is correct.