The correct option (C) 2 : 1
Explanation:
initially capacitance = (A∈0/d) = C
when sheet of thickness t = (d/2) is introduced then capacitance be C'.
and C' = [(A∈0)/(d – t)]
∴ (C/C') = [(d – t)/d] = [{d – (d/2)}/d] ------ thickness is (d/2)
∴ (C/C') = (1/2)
∴ ratio of capacitance is 2 : 1