Which of the following materials is typically used to pack a heat exchanger?

Packing is the correct choice for material typically used to pack a heat exchanger because it serves a critical function in ensuring the efficient operation of the heat exchanger. Packing is designed to fill gaps and prevent leakage between different components, such as between the shell and the tubes where fluids exchange heat. It can be made from various materials, including graphite and other high-temperature resistant substances, to withstand the operational conditions present in a heat exchanger.

This choice emphasizes the need for a material that can accommodate the thermal expansion and contraction that occurs due to varying temperatures, as well as one that can handle the pressure of the fluids involved without degrading. While foam, rubber, and plastic may serve different purposes in other engineering applications, they do not provide the necessary properties, such as durability and resistance to high temperatures and pressures, required in a heat exchanger's packing.