The answer is C.  The key thing to remember about a sentence written in a passive voice, is that the subject is the target, not the agent, of the action.  Thus, answer C is best because the beer got drunk, rather than having done anything itself.  Still, I can see why you might have though answer D was correct, as one could make the argument that the subject ("I") is not doing the action, but commanding someone else to.  Still, a command can never be passive.  Remember that in English specifically, a passive voice is usually marked by the use of the past participle of a verb (to drink --> "drunk"), along with the verb to be (in this case, the verb to get is actually used, but the word "is" could be substituted for "gets" without changing the meaning of the sentence).  Another something to remember is that passive sentences are always somewhat ambiguous, often requiring further explanation.  For example, answer C begs the question of who drinks all that beer every Saturday night.