What should you be able to see in your inside rear-view mirror before returning to your lane when passing on the left?

The best answer is that you should be able to see both headlights of the vehicle you are passing before returning to your lane. This visibility indicates that you have safely cleared the vehicle you were overtaking and that there is enough distance between your vehicle and theirs to return to your lane without risking a collision.

Seeing both headlights suggests that the vehicle you passed is far enough behind you, ensuring that the maneuver was completed successfully and safely. This guideline is crucial for maintaining safe driving practices, as returning to the lane too soon can lead to dangerous situations, such as being cut off by the vehicle you just passed or causing a collision.

The other options, while seemingly relevant, do not provide the same level of assurance that the vehicle has been successfully cleared. For instance, seeing the entire vehicle or just the rear tires may not provide an adequate guarantee of safety, as the vehicle could still be too close for comfort. Additionally, visibility of the driver does not necessarily indicate the vehicle's distance from yours.