Answer
a)$$K_c = \frac{1}{[CO_2]}$$
Work Step by Step
1. Write the expression including all compounds of the reaction:
$$K_c = \frac{[CaCO_3]}{[CaO][CO_2]}$$
2. But, since $CaO$ and $CaCO_3$ appear in the solid state, their concentrations are approximately constant, so they will not appear in the Kc expression:
$$K_c = \frac{1}{[CO_2]}$$
