[sinthreck]

Q: A coordination compound formed from cobalt (II) sulfate and ammonia, after oxidation of the metal ion contains six ammonia molecules which function as ligands. The formula of the compound is properly written as:
a. Co₂(SO₄)₃.6NH3
b. [Co(NH3)₆]2(SO₄)₃
c. Co2(SO4)3(NH3)₆
d. Co₃SO₄(NH₃)₆

I think the answer is (b). but that requires Cobalt(III) ?

[Schrödinger]

The answer is, indeed, (b). And the compound in the answer does contain Co(III). This is obtained from the oxidation of Co(II).
Since the compound contains 6 NH3 molecules as ligands, it has to be an octahedral complex as in b. This is the only option that fits the conditions.