What feature allows the use of a subroutine in a Studio 5000 project?

The ability to use a subroutine in a Studio 5000 project is facilitated through program calls. When a program call is made, it allows the main program to execute a specific subroutine as part of its execution flow. This provides modularity and reusability in the code, as the same subroutine can be invoked multiple times from different parts of the program without the need to duplicate code.

Program calls enable structured programming, making it easier to organize and manage complex control processes within a project. This approach also enhances readability and maintainability, as any changes made to the subroutine are automatically reflected wherever it is called.

In contrast, the other options describe task execution methods but do not directly involve the invocation of subroutines. Continuous Tasks and Annual Tasks refer to different execution cycles for programs but do not inherently facilitate subroutine usage without program calls. Event Tasks are triggered based on events but, again, do not directly address the mechanism of calling subroutines. Therefore, utilizing program calls is the correct method for incorporating subroutines in a Studio 5000 project.