_____ is a set of rules that governs how two computer systems interact.

Concept: A protocol is a formal set of rules and conventions that two or more communicating systems agree to follow. It fixes the syntax (format of the data), the semantics (meaning of each field or message), and the timing/sequencing (order in which messages are exchanged), so that independently built systems can interoperate reliably.

Application: The blank in the question describes exactly this role — a defined framework that governs the format, order, and actions used when two computer systems interact. That description matches the definition of a protocol, so the correct term is Protocol.

Cross-check: Comparing the wording against the remaining options confirms the choice:

• Syntax only fixes the structural rules for writing valid statements in a programming language; it says nothing about how two systems interact.

• Rules is a generic word with no specific technical meaning in networking; it does not capture format, order, and actions together.

• Grammar governs the structure of a language (natural or programming); it does not describe communication between systems.