Step 1: {Relation Between \( K_p \) and \( K_c \)}
The equilibrium constant \( K_p \) and \( K_c \) are related by the following equation: \[ K_p = K_c (RT)^{\Delta n} \] where \( \Delta n \) is the change in the number of moles of gas between products and reactants.
Step 2: {Calculate \( \Delta n \)}
For the given reaction: \[ \Delta n = {moles of products} - {moles of reactants} = 1 - \left( 1 + \frac{1}{2} \right) = -\frac{1}{2} \]
Step 3: {Substitute \( \Delta n \)}
Substitute \( \Delta n = -\frac{1}{2} \) into the relation: \[ K_p = K_c (RT)^{-\frac{1}{2}} \] Thus, the correct answer is (D).