What applies from this traffic sign onwards?

From this sign onwards you are considered to be “inside a built-up area” (town/city limits). That change triggers several special rules.

Your choices 1 and 2 are correct:

Answer 1 is correct because in built-up areas you must not use the horn just to announce an overtake. The horn is basically reserved for warning in danger situations; using it as an “overtaking signal” is not allowed.

Answer 2 is correct because within built-up areas, a parked vehicle may be marked with parking lights (Standlicht) so it can be seen, especially on darker streets.

Answer 3 is not correct because parking lights are not a substitute for proper driving lights. When driving in darkness you must use dipped headlights (and other required lights), not just parking lights. Parking lights are meant for standing/parking, not for normal driving at night.

Rule of thumb: This yellow town sign means “urban rules start now”: horn only for warning, and parking lights are for a parked car—not for driving in the dark.