The introduction of dielectric slab between the capacitor plates increases its capacitance due to

decrease in the electric field

The correct answer is Option (4) → decrease in the electric field

When a dielectric slab is introduced between the plates of a capacitor, the electric field inside decreases due to polarization of the dielectric.

Since $ C = \frac{Q}{V} $ and $ V = Ed $, the reduction in $E$ leads to reduction in $V$ for the same $Q$, hence capacitance increases.

Correct answer: decrease in the electric field