De quo navigavisti? why did you sail? when are you sailing? you are sailing aren't you? from where did you sail?

First of navigavisti is past tense, so you can immediately rule out options B and C. Then, the phrase de quo can be translated as of who, or of what, or from where. Thus, the correct answer is From where did you sail?

De quo means from where, and not why, so A is also incorrect.