
It is given that the current is varying sinusoidally. The current can be written as I=I0sinωt.
The magnetic field through the solenoid B=μ0nI
The flux through the square is ϕ=μ0nIA
The emf is
ϵ=μ0nA×dtd(I0sinωt)⇒ϵ=μ0nAI0ωcosωt
The amplitude of the emf is,
ϵ=μ0nAI0ω=4π×10−7×10−250×4×10−4×2.5×700⇒ϵ=44×10−4V