Answer: D. Control rods absorb neutrons, so inserting them further reduces the number available to sustain fission and lowers the fission rate. Fission products in removed fuel are radioactive, so the fuel requires long-term storage.
A: confuses shielding, which limits radiation escaping the reactor, with control of the neutron population driving fission.
B: correctly controls the rate, but shielding does not remove the need for long-term storage of radioactive fuel.
C: withdrawing control rods absorbs fewer neutrons, tending to increase rather than decrease the fission rate.
Remember: control rods regulate fission; shielding limits radiation exposure, while radioactive waste needs long-term management.