This is a punctuation mark that is used between clauses of a compound sentence when a conjunction is not used, before conjunctive adverbs that join independent clauses, and in a series when the series already contains commas.

Which of the following factors most consistently contributes to the speaker’s believability? a the age and experience of the speaker b the physical environment in which the speech is delivered c the sincerity, knowledge, and trustworthiness of the speaker d the position of the thesis statement in the speech