The correct option is A 0
How do we start here?
We know that potassium has a +1 charge always, so the negative ion i.e. the complex would have a total charge of -4 to balance out the 4 K+ ions.
Let's look at the compex now. The complex has CN as well as Ni and as you know, the cyanide ion would have a charge of -1. So the entire complex needs to have a negative 4 charge, we can write this equation then: (x is the oxidation state of nickel here)
x+4×(CN−)= charge of the anionic complex
x+4×(–1)=–4 ;
which gives the value of x to be 0.