Which of the following hand signals would a driver use to indicate that s/he is turning left?

Using a left hand and arm straight out indicates that a driver intends to turn left. This hand signal has been standardized and is easily recognizable to other drivers and pedestrians. By extending the left arm straight out from the vehicle, the driver is clearly communicating the intention to make a left turn, which is important for ensuring safety and maintaining clarity in traffic. This approach is part of safe driving practices, as it provides visual cues to others on the road about the driver's intentions.

In contrast, raising the left arm upward signals a right turn, while bending the arm downward indicates a stop or slow down. The option involving the right arm extended straight out actually indicates a left turn, which could lead to confusion. Therefore, using the correct hand signal for a left turn not only adheres to the rules of the road but also promotes responsible driving behavior.