What do you understand by ‘dielectric constant’ of a material medium?

When a medium of dielectric constant K is introduced between the plates of a parallel-plate capacitor, how do the capacitance change ?

When a medium of dielectric constant K is introduced between the plates of a parallel-plate capacitor, how do the PD between the plates change?

A parallel-plate capacitor having vacuum between the plates has capacitance C. What will be the capacitance When the distance d between the plates is doubled.

A parallel-plate capacitor having vacuum between the plates has capacitance C. What will be the capacitance When a sheet of thickness t and dielectric constant K is placed between the plates? (Assume r = d.)

Name a few dielectrics commonly used in capacitors.