Select the option that can be used as a one-word substitute for the given group of words.

The act or habit of talking in one's sleep

Somniloquy

The word that can be used as a one-word substitute for "the act or habit of talking in one's sleep" is somniloquy.

• Soliloquy is a speech delivered by a character to oneself, as if no one else is present.
• Prologue is an introductory speech or scene that precedes the main part of a play, film, or other work.
• Monologue is a speech delivered by a single character.
• Somniloquy is the act or habit of talking in one's sleep.

Somniloquy is the most appropriate word for the given group of words because it specifically refers to talking in one's sleep. It is derived from the Latin words somnus, meaning "sleep," and loqui, meaning "to speak."