(i)-(c), (ii)- (a), (iii)-(b)

(i)-(c) Osmium has the highest oxidation state of +8 because it can expand it’s octet and use all its 8 electrons.

Electronic configuration of Os- [Xe] 4f¹⁴ 5d⁶ 6s²

5d⁶ and 6s² can be excited to get an oxidation state of +8

(ii)-(a) Manganese can show upto +7 oxidation state

(iii)-(b) 3d block element with highest is Cr.