The correct option is
C energy decreases
→ When the capacitor is charged and battery is removed the charge present on capacitor remains constant. (Law of conservation of charge)
→ We know
C=ε0AdWhen slab is introduced,
C=kε0AdC increases and
Q is constant.
We know:
Q=CV ∴ V has to decrease.
We know energy store in capacitor =12QV
∴ as V decreases, energy decreases.