Which of the following is a register in communication?

In the context of communication, the term "register" refers to the level of formality, style, tone, and vocabulary used in a particular communicative situation. The correct answer, informal, specifically describes one of the registers that characterizes communication that is relaxed, casual, and often used in interpersonal settings.

Informal register typically includes colloquial language, slang, and a more relaxed syntax that is suitable for everyday conversations among friends or peers. This register reflects a friendly atmosphere where the norms of high formality or professionalism are not necessary.

The other options represent different concepts or levels of communication that do not distinctly fit the definition of register. For example, while hierarchical might suggest a structure of communication based on status, it does not specifically denote a communication style. Casual is somewhat similar to informal but lacks the specific context that illustrates the communicative formality level. Cosmopolitan, while it refers to a worldly and sophisticated style, does not define a communication register like informal does. Thus, informal stands out as the most accurate representation of a register in communication.