When passing another vehicle, it is important to ensure that you have safely cleared the vehicle before returning to your lane. The safest method to determine this is by confirming that you can see the tires of the vehicle you just passed in your rearview mirror. This indicates that there is sufficient distance between your vehicle and the vehicle you have overtaken, minimizing the risk of a potential collision as you merge back into your lane.
The other options do not provide the same level of certainty and safety. For instance, determining the safe distance based solely on a time basis, such as two seconds ahead, can be less reliable due to varying speeds and conditions. Similarly, measuring your rear bumper's position in relation to the other vehicle, such as being 10 feet ahead, doesn't take into account the actual visibility needed to ensure a safe return to the lane. Thus, being able to visualize the other vehicle's tires in the rearview mirror offers the clearest indication that you are safely ahead to merge back into your lane.