Answer:
0.7969 mm
Step-by-step explanation:
Capacitance of the capacitor, C = 3 nF = 3 x 10^-9 F
Area of plate, A = 0.27 m^2
Separation between the plates is filled with air.
Let the separation between the plates is d.
Use the formula for the parallel plate capacitance which is given by

where, C be the capacitance, εo is the permittivity of free space or air, d be the separation between the plates and A be the area of plate.
The value of εo = 8.854 x 10^-12 C^2 / Nm^2
By substituting the values in the above expression, we get

d = 7.969 x 10^-4 m
d = 0.7969 mm
Thus, the separation between the plates is 0.7969 mm.