Chapter 23 - Magnetic Flux and Faraday's Law of Induction - Problems and Conceptual Exercises - Page 835: 100

$(C)\ 8.5\times 10^{-5}T$

We know that $|\epsilon=N|\frac{(B_2-B_1)\pi r^2}{\Delta t}|$ This can be rearranged as: $B_2=B_1+\frac{|\epsilon|\Delta t}{N\pi r^2}$ We plug in the known values to obtain: $B_2=(1.2\times 10^{-5})+\frac{(8.1\times 10^{-4})(0.38)}{3\pi(0.67)^2}=8.5\times 10^{-5}T$