When a parollel plate capacitor is connected with a battery, the potential drop generated accross the capacitor is same as that of piotential drop generated across battery. i.e Potential drop is constant 

When a slab of dielectric material is introduced between the plates of capacitan then its capacitance will change and hence charge well also change

Let, initial copacitance $=C$ and charge $=Q$

After introducing slab Copacitance $=K C$ and charge $=q$

Since,

$E_1=E_2$

$\frac{Q}{C}=\frac{q}{K C} \quad \Rightarrow q=K Q$

Sinc, $k$ is always greater than 1 for all dielectric materials

$\therefore q > Q$

Hence charge on the plates in comparsion with earlier charge is more