Answer
Choice E.
Work Step by Step
The strong force does result from the exchange of $\pi$ mesons between the neutron and the proton. This is correct on the nuclear scale, as discussed on page 923. Pion exchange explains the strong nuclear force, as choice A says.
However, suppose we look at a much smaller scale, and consider the individual quarks that comprise the proton, the neutron, and the $\pi$ meson. As explained on page 937, the transfer of gluons explains the strong force between quarks, as choice D says.
Choices A and D are both correct, at different scales, which is what choice E says.