The correct answer is A. inevitable.
Inevitable means certain to happen; unavoidable. In this case, the bus was at full speed and its brakes failed, so an accident was certain to happen.
B. infallible means incapable of making mistakes. This is not the correct answer because the bus driver made a mistake by not checking the brakes before driving.
C. essential means necessary or very important. This is not the correct answer because an accident is not necessary or very important.
D. undeniable means impossible to deny or dispute. This is not the correct answer because it is possible to deny or dispute that an accident was inevitable. For example, someone might argue that the bus driver could have avoided the accident if they had been driving more carefully.