The level of courtesy that exists most useful fits the subject and the audience.
The speaker of a speech should recognize his audience and specify which language exists most suitable for that audience. It would be extremely inconvenient if a speaker utilized informal language in an academic-level speech at a university, also it would be equally unsuitable to utilize formal language full of technical phrases for kindergarten children. From this, we can say that an essential style determination a speaker produces when composing a speech exists to determine the level of formality that nicely fits the topic and audience. The level of courtesy that exists most useful fits the subject and the audience.
Therefore, the correct answer is option D. level of formality.
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