At which traffic sign at the next junction do you generally have right of way?

You answered correctly because the first sign indicates that you are approaching a junction where you have priority over traffic coming from the side roads. This sign warns you that others must yield to you.

The second sign, a black X inside a red triangle, warns of an upcoming intersection without priority—traffic from all directions must pay attention and often yield according to right-of-way rules.

The third sign shows a blue square with arrows indicating that on a narrow road, oncoming traffic has priority (white arrow going forward), and you must yield (red downward arrow).

In summary, the first sign means you generally have right of way at the next junction, which is why your answer is correct according to German traffic rules.