Final answer:
The type of speech that should be relatively brief is an introduction. It engages the audience and outlines the main points to set the stage for the rest of the speech, and it must accomplish this concisely. Option B is the correct answer.
Step-by-step explanation:
The question addresses the appropriate length of different types of speeches. Among the given options (presentation, introduction, persuasive, commencement), the type of speech that should be relatively brief is the introduction. A successful introduction sets the stage for the speaker to present their main ideas and establish credibility without overstaying its welcome.
It engages the audience, presents the topic, validates the speaker's credibility, and leads into the main theme with a clear thesis statement. Unlike the other options, which might require more extensive content to inform, persuade, or commemorate, the introduction serves as a concise and potent beginning to a longer discourse.
Given the composition of various speech types, it is pertinent to focus on the speech's aim, be it to inform, entertain, or persuade. Choices about language and delivery should consider the audience and the cultural context within which the speech occurs, allowing the speaker to effectively challenge convention while staying relatable. Thus, while other types of speeches have their own distinct features and necessary lengths, the introduction's brevity remains a hallmark of its function and purpose.
In conclusion, the correct option is b. Introduction.