Which of the following best describes a primary function of a butterfly valve?

A butterfly valve is primarily designed to control flow by providing a means to adjust the size of the flow passage, which allows for throttling of the fluid. The valve consists of a disk that rotates around a central axis, enabling it to either completely open, partially open, or close. This rotational movement allows operators to fine-tune the flow rate and pressure of the fluid passing through the valve.

While a butterfly valve can also serve other purposes like isolating flow when fully closed or preventing backflow if installed correctly with a suitable design, its main characteristic that distinguishes it in applications is its ability to regulate flow effectively. The design is particularly efficient for throttling due to the minimal pressure drop when the valve is partially open, allowing for precise control over the flow rate.