Which road sign indicates that turning around is not allowed?

All three signs in this question indicate situations where turning around (U-turns) is not allowed in Germany.

The first sign explicitly shows a U-turn with a red prohibition line and directly means "no U-turn." The second sign marks the entrance to the Autobahn (motorway), where making a U-turn is not allowed for safety reasons, as all traffic must travel in the same direction. The third sign indicates the entrance to a tunnel, and turning around is also strictly forbidden inside tunnels for safety.

So, whenever you see any of these signs, turning around is either directly prohibited by the sign (as in sign 1), or by traffic law due to the road conditions (as on the Autobahn and in tunnels). Remember: on motorways and in tunnels, U-turns are always prohibited, even if no explicit prohibition sign is posted.