I am not sure if I have done the following problem correctly. To start, I do not know how to prove the electric field must be directed radially. Assuming this is true, then the magnitude of the field must be uniform along any spherical boundary (otherwise we could form a nonconservative loop that straddles this boundary). If we then apply Gauss's Law for dielectrics on a spherical surface of radius r, we have: 2 pi r2(K E + E) = Q/e0 or E = 1/(2 pi e0) Q/(1+K)r2. From this it easy follows Vab = 1/(2 pi e0) Q/(1+K) (1/a - 1/b) and then C = 2 pi e0 (1 + K) (a b)/(b - a). On reflection this does at least make some sense since the capacitance rises with stronger dielectrics and reduces to the standard expression when K = 1. But the wording in part B implies the field strength is not the same in the lower and upper portions of the capacitor. And how one shows the field is directed radially throughout troubles me (though I can't imagine it being much else since it must at least be radial near the conductors).