The oxidation state of a central metal is obtained by setting the sum of all charges in the species equal to its overall charge. Potassium hexafluoridocobaltate is K3[CoF6]. Each potassium ion carries +1, and three of them contribute a total of +3, balancing the charge of the complex anion, which must therefore be -3. Inside that anion six fluoride ligands each carry -1, contributing -6. Letting the cobalt oxidation state be x, we write x + (6 × -1) = -3, so x - 6 = -3 and x = +3. Hence cobalt is in the +3 state. The value +2 ignores the correct anion charge, +4 wrongly assumes a -2 complex, and +6 absurdly cancels the fluoride charges entirely. This balance method is the routine NCERT approach to oxidation states in coordination compounds, and here it also explains why the complex is a high-spin fluoride species. Plausibility check: cobalt(III) is a very common and stable oxidation state in octahedral complexes, so the assignment is chemically reasonable.