While all the options contribute to *some* aspect of a persuasive speech, strong writing and powerful delivery are the *key* elements that determine its effectiveness. A speech can have impeccable research (c) but fail if it's poorly written or delivered. Similarly, informal word choice and grammar (d) are generally detrimental to persuasive speech. Drafting and revision (a) are important steps in the process, but they're means to the end of achieving strong writing. The core of a persuasive speech lies in its ability to connect with the audience through both the content (strong writing) and the presentation (powerful delivery).