Answer:
Marian washed her car last week.
Why?
"last week" is talking about the past. Therefore, the word should be whatever is in past tense.
<em>Washes - present</em>
<em>Washed - past</em>
Thus, the answer is Washed
Also, if you're not sure, just say them out loud. Whichever one sounds correct, put down as your answer.
:D