Which command would make Karel turn around?

The command that makes Karel turn around is to execute two successive left turns. Each left turn rotates Karel 90 degrees to the left. Therefore, when Karel turns left once, he faces a new direction, and after a second left turn, he completes the full 180-degree turn, facing the opposite direction from where he started.

This is a fundamental aspect of Karel's movement commands. Using two left turns is the most straightforward method to achieve a turnaround, as it directly utilizes the defined movement capability without requiring any additional moves or complications. Other answer choices either involve a single turn or movement in a way that does not achieve a full turnaround.