What do you have to do in this traffic situation?

With a STOP sign in Germany you must always come to a complete standstill, even if you think the road is clear. If there is a stop line, you stop at the stop line. If there is no stop line, you stop at the “line of sight” (Sichtlinie), meaning the point where you can actually see the cross traffic safely. Often that means stopping first before the intersection (so you don’t roll into it), then creeping forward and, if needed, stopping again at the best viewing point.

So answer 1 is correct: stop at the line of sight and give way. Answer 2 is wrong because the duty to stop comes from the STOP sign, not from a painted stop line. Answer 3 is wrong because you don’t stop only when there is cross traffic; you must stop every time, then yield to cross traffic and also watch for pedestrians/cyclists before entering or turning.