The oxidation number of cobalt in K₃[Co(NO₂)₆] is:

+3

The correct answer is option 2. \(+3\).

The given complex is \(K_3[Co(NO_2)_6]\)

The oxidation number of \(K\) is \(+1\)

The oxidation number of the \(NO_2^-\) ligand is \(-1\).

Let the oxidation number of \(Co\) be \(x\)

Since, the complex is neutral, so the total charge is \(0\).

Thus,

\(3(+1) + x + 6(-1) = 0\)

\(⇒ +3 + x - 6 = 0\)

\(⇒ x - 3 = 0\)

\(⇒ x = +3\)