Explanation: in this case the only possible option is "prevent" because the main idea of the sentence relates to "precautions" and "accidents", so it means that the author wants accidents not to happen.
Option A would sound incoherent as precautions are not meant to create accidents. Option B is not possible as the verb "ensure" implies to make something happen for sure and accidents just occur unexpectedly.
Option D is also not possible because accidents cannot be interrupted by being cautious, they either happen or not.
