Final answer:
When a passing vehicle signals to cut in front, you should let them merge safely, adjusting your speed or changing lanes if necessary.
Step-by-step explanation:
When a passing vehicle signals that they must cut in front of you, the appropriate response is to let them merge safely. You should adjust your speed or move to another lane to create space for the merging vehicle. It is important to follow the rules of the road and be courteous to other drivers.