Answer
$\text { (d) } 4$
Work Step by Step
Recall Gibbs phase rule. For a system: $$πΉ=πΆβπ+2,$$ where
$F$ = degrees of freedom (independent variables)
$C$ = number of components
$P$ = number of phases
In our case $$F=3-1+2=4.$$ The correct answer is $\text {(d) }$.