Final answer:
The correct hand signal for a left turn is to extend the left arm and hand straight out to the side.
Step-by-step explanation:
The correct hand signal for a left turn, when driving or cycling, is D. left arm and hand extended straight out. This is a universally recognized signal that demonstrates the intention to make a left turn. Extending your left arm straight out to the side of your body is the simplest way to indicate this direction clearly to other drivers and pedestrians. On the other hand, a left arm and hand pointing up would signal a right turn, and pointing downward typically indicates the intention to stop or slow down.