The correct answer is A) expansion joints in the condenser shell. When condenser tube ends are rolled into both tube sheets, the different rates of material expansion between the tubes and tube sheets can cause stresses and leakage. To compensate for this, expansion joints are installed in the condenser shell to allow for the differential expansion, relieving the stress on the tube-to-tubesheet joints. The other options are incorrect because: B) metallic packing would not compensate for the differential expansion, C) threaded brass ferrules would not provide the necessary flexibility, and D) belled joints at the tube ends would not address the issue of differential expansion between the tubes and tube sheets.