Chapter 16 - Electric Potential, Energy, and Capacitance - Learning Path Questions and Exercises - Multiple Choice Questions - Page 590: 20

(c) it is doubled

$C=\frac{\kappa\varepsilon_{0}A}{d}$ When $d'=\frac{d}{2}$, $C'=\frac{\kappa\varepsilon_{0}A}{\frac{d}{2}}=2\frac{\kappa\varepsilon_{0}A}{d}=2C$ When the distance of separation is cut in half, the capacitance is doubled.