What should you ensure before moving back into your original lane after passing?

To safely move back into your original lane after passing another vehicle, it is essential to ensure that you can see both headlights of the vehicle you just passed. This is a critical safety measure as it indicates that you have sufficiently cleared the other vehicle's front end. If both headlights are visible in your rearview mirror, it confirms that there is enough space between your vehicle and the one you just passed, allowing for a smooth transition back into your lane without risking a collision.

The other options do not encompass this fundamental aspect of spatial awareness needed during lane changes after passing. Simply confirming the lane is clear does not guarantee that you are safely ahead of the passed vehicle. While it is important that the lane is clear and that turn signals are used for signaling intention, the visibility of both headlights directly addresses the crucial aspect of ensuring your own safety and the safety of the vehicle you just passed.