List of top Chemistry Questions on Chemical equilibria asked in GATE ES

In the reversible reaction shown below, \(k_f\) is the forward reaction rate constant and \(k_r\) is the reverse reaction rate constant.
The CORRECT expression representing the reaction equilibrium constant is ______.
\[A + B \underset{k_r}{\overset{k_f}{\rightleftharpoons}} C + D\]