What action should Karel perform to face the opposite direction?

To face the opposite direction, Karel needs to perform a specific series of movements. When Karel turns left, it rotates 90 degrees to its left. If Karel were to turn left twice, it would end up facing the opposite direction from the original position. The first left turn changes its direction by 90 degrees, and the second left turn changes it another 90 degrees, totaling 180 degrees, which consists of turning around completely. Thus, turning left twice successfully aligns Karel to face the opposite direction.

While other options might visually seem to change Karel's position, they do not necessarily result in facing the exact opposite direction. Moving backward would simply continue in the same direction without altering Karel's facing direction. Turning right once would change the direction to the right but not to the opposite. Turning around is another way to describe facing the opposite way, but in the context of options provided, turning left twice is the most direct way to achieve the desired outcome.