Signs that have a white background and a red circle with a slash through it indicate what?

Signs featuring a white background combined with a red circle and a slash indicate prohibitive actions. This visual format is universally understood as an instruction to not enter or partake in the indicated action. In this case, the sign would typically represent "no U-turns," which means that making a U-turn in that area is not allowed.

While the signs for "no left turns" and "no right turns" share a similar design and serve the same prohibitive function, the particular question focuses on the specific meaning of the stated sign. Each of these prohibitive signs is distinct, and though they all fall under the category of prohibiting certain driving maneuvers, "no U-turns" is explicitly what this sign communicates. Hence, while all the options represent actions that could be prohibited, only the first choice aligns directly with the specific interpretation of the described sign.