Please note that I am not asking how to do this question. It is not homework. :) This is part of an externally set exam that some of my students sat, and with which I am disappointed with the "officially" correct solution. To me the solution is reasonably straight forward, however many people seem to disagree with me, in particular those who wrote the "official" answers by which this was marked. I won't give it away, but the "officially" correct answer (which as you may have guessed, I believe is wrong) seems to display a poor understanding of Lenz's law. Can you spot the correct answer and also the other one, which (if given by a student) you might well understand how they made that mistake, but would not expect anyone who really understands this stuff to make. What do you think is the correct answer?