What must be true about the programming language used for a routine?

For a routine in a programmable logic controller (PLC) environment, it is essential that it is programmed entirely in the same language. This uniformity ensures consistency in syntax, semantics, and structure, which is crucial for clarity and maintainability. In a PLC context, mixing different programming languages can lead to confusion and errors, as each language may interpret commands and instructions differently.

Using a single language throughout a routine also facilitates easier debugging, testing, and integration, allowing programmers and engineers to focus on solving problems instead of managing the inherent complexities that arise from language diversity.

While other options may have merit in different programming contexts, maintaining a singular language approach is paramount in ensuring effective routine execution within PLCs.