As we know that capacitance of parallel plate capacitor is given as
[tex]C = \frac{\epsilon_0 A}{d}[/tex]
here we know that the gap between the plates of capacitor is filled by air or vacuum
now if we fill the space between the plates by dielectric medium then we will have
[tex]C_{med} = \frac{k \epsilon_0 A}{d}[/tex]
so it will increase the capacitance of capacitor
Also we can see that capacitance is inversely depends on the distance between the plates of capacitor plates
So capacitance will increase if we decrease the distance between two plates
so correct answers are
D) introduce a dielectric material between the plates
E) decrease the separation between the plates