When driving in the left lane of a four-lane highway, when should you use your turn signal?

Answer: C — When merging out of the lane

Using your turn signal when merging out of the left lane is important for communication and safety. Option A is incorrect because you should use your turn signal when merging into the left lane, not when merging out of it. Option B is incorrect because you should only use your turn signal when making turns, not when driving straight in the left lane. Option D is incorrect because using a turn signal does not indicate a change in speed, rather it signals a change in direction. Therefore, the correct answer is C - when merging out of the left lane.