Cover letter and resume. Which is more reasonable. You should be reading the job ad closely to make sure you match that. Then do some research on the company to make sure you understand what they do - often you can think of some other relevant experience that isn't in the ad.

Don't forget that this isn't a hackernews comment. You should take an hour (day is better!) break and then reread and revise everything. Don't send the rough draft or even the second draft in - follow the whole writing process they told you in school. Some people will spot a spelling error at the bottom of the page in one second and reject you, so make sure everything is perfect.