Which type of computer is designed for use by one person at a time?

The correct choice is a personal computer, commonly referred to as a PC. Personal computers are specifically built for individual use, allowing one person to operate the computer at a time. This class of computers includes desktops, laptops, and tablets, and they are designed to perform a variety of tasks such as document creation, web browsing, gaming, and more.

In contrast, workstations are high-performance computers tailored for tasks that require more power, such as graphic design or data analysis, and while they are used by one person at a time, they typically serve specialized applications. Mainframes are powerful machines designed for large-scale computing tasks and support multiple users simultaneously. Servers are designed to manage network resources and provide services to multiple users or devices rather than being tailored for an individual user.

Thus, the functionality and design of personal computers distinctly cater to sole usage, making them the best fit for the question.