Answer
$\text { (d) } 4$
Work Step by Step
We are given $8$ species with $4$ elements. The number of equilibrium constant relations is $$8-4=4.$$The answer is $4$ relations because with $8$ chemical species built from $4$ elements, there are $4$ independent reactions that can be written, and each one gives an equilibrium constant expression necessary to solve for the composition.
The correct answer is $\text {(d)} $.
