The correct answer is A. Word Choice
Explanation:
In speeches, the speaker's tone can be defined as his/her attitude towards the main topic, for example, the speaker's attitude towards a speech about global warming can be optimistic, pessimistic, serious or ironic. The tone is defined by the use of the language or the word choice because the speaker's feelings, perspective, and perception are revealed through the words the speaker uses. For example in the case of global warming saying "Global warming is an issue we must address" shows a different attitude than saying "Global warming is an issue other want us to believe". Thus, the element that most helps establish a speaker's tone is word choice.