Final answer:
The function of a brake drum or rotor in a vehicle is to act as a heat sink, absorbing and dissipating the heat produced by the friction during braking.
Step-by-step explanation:
The function of a brake drum or rotor is to act as a heat sink and a friction element. It is designed to absorb and dissipate the heat generated by the friction between the brake pads and the rotor during the braking process. As a car moves, its kinetic energy is converted into heat when brakes are applied. The brake drum or rotor has to withstand high temperatures and ensure that the energy conversion process does not lead to brake failure. The correct answer to the given question is B: a heat sink, as it primarily serves to absorb and dissipate heat, though it is also inherently a friction element.