What is a safe indicator that you can return to your lane after passing a vehicle?

The correct response is based on the principle of ensuring that you have completely passed the vehicle before returning to your lane. When you can see both headlights of the car you have just overtaken in your rear view mirror, it demonstrates that you have cleared the vehicle's length and are at a safe distance within your lane. This visibility is crucial because it confirms that there is sufficient space between your vehicle and the one you have passed, reducing the risk of a collision when you merge back into your lane.

The other choices do not offer similarly reliable indicators for safe lane changes. A vehicle's turn signal indicates its intended direction but does not confirm that it is safe for you to return to your lane. The speed of the other car is irrelevant to your immediate safety in this context. Lastly, the absence of vehicles in front of you might signal a clear path ahead, but it does not ensure that it is safe to merge back into your lane after passing. If the overtaken vehicle isn't completely clear of your path, returning could lead to an unsafe maneuver.