The correct answer is A. determine.
To ascertain something is to find out the truth about it. In this case, the truth is the actual cause of the accident. The enquiry was ordered to find out what caused the accident so that it could be
prevented from happening again.The other options are incorrect because they do not accurately describe the purpose of the enquiry. To investigate something is to look into it carefully in order to find out the truth. To discover something is to find it out for the first time. To prove something is to show that it is true.
In this case, the enquiry was not ordered to investigate the accident, discover the cause of the accident, or prove the cause of the accident. It was ordered to determine the cause of the accident.