Given that,
Area A=A2
Separation between the plates =d
Constant current =i
Now, the electric field between plates of the capacitor
E=qε0A
Now, the flux through the area
ϕE=qε0A×A2
ϕE=q2ε0
Now, the displacement current is
Id=ε0×dϕEdt
Id=ε0×d(q2ε0)dt
Id=12(dqdt)
Id=i2
Hence, the displacement current is i2