Final answer:
When changing lanes, preparing to pass another vehicle, or entering traffic, it is important to first use your mirrors and then check for oncoming traffic. Therefore, the correct answer is B. Check for oncoming traffic.
Step-by-step explanation:
When changing lanes, preparing to pass another vehicle, or entering traffic, it is important to signal and check for passing traffic. After using your mirrors, the next step is to check for oncoming traffic. This ensures that no vehicles are approaching from the opposite direction that may impede your lane change or entry. Checking the rear of your vehicle (option A) is important to ensure that there is no immediate danger behind you, but it does not provide information about the traffic coming towards you from the front. Checking the front of your vehicle (option C) is not necessary in this situation as the focus should be on the traffic that may affect your lane change or entry. Checking your vehicle's blind spots (option D) is also an important step to ensure there are no vehicles in your blind spots before changing lanes or merging, but it should be done after checking for oncoming traffic.