Write the compound and identify ionic parts

The compound is $K_4[Ni(CN)_4]$

Since potassium is an alkali metal, each $K$ has oxidation state $+1$.

Find the charge on the complex ion

There are $4$ potassium ions: $4 \times (+1) = +4$

Because the whole compound is neutral, the complex ion must have charge $-4$: $[Ni(CN)_4]^{4-}$

Assign oxidation state to ligands

Cyanide, $CN^-$, is a monodentate ligand with charge $-1$.

For $4$ cyanide ligands: $4 \times (-1) = -4$

Let the oxidation state of Ni be $x$

Then for the complex ion: $x + 4(-1) = -4$ $x - 4 = -4$ $x = 0$

Evaluate the options

• A: $0$ ✅
• B: $+1$ ❌
• C: $+2$ ❌
• D: $-1$ ❌

Final answer

The oxidation state of nickel is: $\boxed{0}$