Welcome Nate. Great to hear that you've rolled up your sleeves and are working through a UX program.
You're absolutely correct in your observation that one of the core principles behind user-centred design is making informed design decisions. Much like in your school maths test, designers should be able to defend the decisions they make, and a good way to do that is to be transparent in showing how you arrived at a certain conclusion. Sorry, there's no multiple choice in UX! There's actually never any right answer either, just a bunch of data (people), techniques, insights, and deductions to guide you on your way.
Definitely have a read through our [process and [URL="http://uxmastery.com/resources/techniques/"]techniques pages. There are a bunch of videos(http://uxmastery.com/resources/process/) that we've published about these techniques too, that you might find useful.