The correct answer is C) heat, or cool one fluid by means of another fluid. The primary purpose of a heat exchanger is to transfer heat from one fluid (liquid or gas) to another fluid, either to heat up or cool down one of the fluids. This is achieved by bringing the two fluids into close contact, allowing heat to be transferred between them without the fluids mixing. This is a fundamental concept in the design and use of heat exchangers on board vessels. The other answer choices are incorrect because they do not accurately describe the general purpose of a heat exchanger. Maintaining steady pressure (A), eliminating hot air from the condenser (B), and reducing engine room temperature (D) are not the primary functions of a heat exchanger, although they may be secondary effects in some applications.