Copper is a metal (conductor) with an infinite dielectric constant ($K \to \infty$).

The capacitance of a parallel plate capacitor with a slab of thickness $b$ and dielectric constant $K$ is:

For a metallic conductor like copper, $K \to \infty$, which gives $\frac{b}{K} \to 0$:

The effective separation between the capacitor plates is reduced from $d$ to $(d - b)$. Hence, option 2 is the correct answer.