What does a 'No U-Turn' sign prohibit?

Answer: B — Performing a U-turn at that location.

A No U-Turn sign means you’re not allowed to reverse your direction at that spot. A U-turn is turning around in one smooth motion to go the opposite way. So, when you encounter this sign, you should not attempt a 180-degree turn there; you can still proceed straight or make other permitted turns, as long as those moves are allowed by signals and other signs. The sign doesn’t automatically ban parking or left/right turns, unless another sign specifies such restrictions. If you need to change direction, you’d do so at a location where a U-turn is permitted.