What applies to this road that is outside of a built-up area?

You answered correctly that the rule applies to performing a left turn on this road outside a built-up area.

In German traffic rules, outside built-up areas (in the countryside), left turns are generally allowed only at intersections or designated places. However, on multi-lane roads such as this one, you must be especially cautious with left turns because of higher speeds and traffic volume.

The other options—overtaking and conditions involving traffic jams—are not the key considerations for this question. Overtaking rules depend on road markings and signs, and handling traffic jams involves different regulations.

Remember, when driving outside built-up areas, you must adhere strictly to turning rules for safety and smooth traffic flow. Left turns are permitted, but only where it is safe and legal, according to the road layout and signs.