Rowley,
As stated by Alex, all students taking the exam will get the marks for this question.
Although I said that dihedral would tend to cause wing twist I do not believe that this is the option the examiners were looking for.
To assess any possible twisting effects we should imagine that we have made vertical chordwise cuts through the wing roots and fixed the wings back on using a single horizontal pin. Now imagine the effects of the drag at the tips of dihedral wings. The tips are higher than the horizontal pin, so the drag will tend to cause the wings to rotate leading upwards. The lift at the tips of swept back wings will have a similar effect, but tending to rotate the wings leading edge down. This is why swept back wings are less prone to divergence. As the lift bends them upwards it also twists the tips leading edge down, thereby reducing the lift at the tips. This reduces the bending effect, so the wings do not diverge.
But the degree of twisting is proportional to the amount by which the tips are above and behind the roots. Lots of aircraft have 20 degress or more of sweepback, but dihedral angle is usually only a few degrees. this coupled with the fact that lift is much greater than drag, means that the torsional effects of sweep back are much greater than those of dihedral.