When monolingual speakers move from formal to informal modes of speech within the same spoken or gestured idea, they are engaging in :

The correct answer is Code-switching.

Explanation:

Code-switching refers to the practice of alternating between two or more languages, dialects, or speech varieties (such as shifting between formal standard language and informal casual speech) within a single context or conversation.

Even among monolingual speakers, shifting between different registers (e.g., moving from a formal academic tone to an informal conversational style) while conveying the same idea is considered a form of code-switching or register shifting.

Breakdown of other options:

• Code-mixing: Refers to the hybrid mixing of two or more languages or codes within a single sentence or clause, often without a change in topic or register.

• Dialects: Varieties of a language characteristic of a particular group of speakers or regional population.

• Sociolinguistics: The academic study of the relationship between language and society.