Given: B=0.5T\text{at}t=0&B=0, at t=0.5s.
Magnetic field will decrease linearly as given below. We can say, value of magnetic field at t=0.25will beB=0.25.

Therefore,
ϵind =∣ΔtΔϕ∣=∣ΔtΔ(BA)∣=A∣ΔtΔ(B)∣
=π×(π10)2×10−4×(0.250.25)
=10−2×1
=0.01V=10mV