Final answer:
The assumption that speakers and listeners act unitedly and rationally is best described in the Cooperative Principles, which are part of pragmatic theory and stipulate that conversations follow certain maxims of quality, quantity, relevance, and manner for effective communication.
Step-by-step explanation:
When we assume that speakers and listeners act unitedly and rationally, this concept is best described in the B. Cooperative Principles.
These principles, as formulated by philosopher H.P. Grice, imply that participants in a conversation generally adhere to four maxims: quality, quantity, relevance, and manner. The assumption here is that both speakers and listeners are cooperating to make the conversation as informative, truthful, relevant, and clear as possible. While pragmatics does deal with the use of language in social contexts and the impacts this can have on meaning, the Cooperative Principles specifically outline how rational conversation is expected to proceed.
The Cooperative Principles are part of pragmatic theory, which suggests that truth can be shaped by practical consequences and social functions. These principles are crucial when considering rationality and united action in conversations and interactions. Understanding how language functions within groups to construct reality is pivotal in the study of pragmatics and the application of these principles.