Hello, i am here to help you according to your query.

The effective separation between the plates with air in between is

2d-t+\frac{t}{k}

that is

10-5+\frac{5}{10}=5.5 mm

(i)   new capacitance

C'= \frac{\epsilon_0A}{5.5mm}=\frac{10}{11}C

\approx 182\mu F

(ii) change in the electric field or effective new electric field

=\frac{100v}{5.5\times 10^{-3}m}

\approx 18182\; v/m

(iii) Energy density

Energy=\frac{1}{2}CV^2

As C has decreased energy also decreases, so energy density also decreases.

Hope,this information will helpful to you.