Let me explain Question 2 in test 45. The correct answer is: The reason for this is (this means simply ‘because’) that the Fiesta (the name of a make of Ford car) is preventing (stopping) our delivery lorry from leaving the premises (the site). In other words the car is in the way and the lorry can’t get out.
You could probably get away with using ‘keeping’ but it wouldn’t be the best of the possible answers.
In any multiple choice questions you have to choose the best of all the alternatives.
The reason why I chose ‘prevent’ in this test rather than ‘keep’ is principally because of the force of the verb indicating that the car is stopping the lorry from leaving. ‘Keep’ in the sense of ‘obstruct’ is more associated with animate subjects since it indicates that there is some kind of intentional obstruction as in:
The athlete in the race was accused of unfairness because he was deliberately keeping the other members of the team from overtaking him.