You should check the efficiency of your bus's brakes before...?

Answer: B — Joining the highway

It is important to check the efficiency of your bus's brakes before joining the highway because this is when you will be traveling at high speeds and need to rely on your brakes to control the vehicle. Checking the brakes before each trip, while still a good practice, may not necessarily catch any issues that could arise when driving on the highway. Driving at night and stopping for fuel do not have any relevance to checking the brakes.