As far as I can see, the attempts to justify answer C are grasping at straws. It is possible that the problem was incorrectly worded, and that it was intended to say that the incident has a 30% probability of happening in each of the last three months. That intention would make C correct, but it is not what is expressed by the actual wording, which doesn't say anything about "each". The only plausible interpretation of the given information, in my opinion, is that 30% is the probability that an incident (at least one incident) will occur in the three-month period that spans the third, fourth, and fifth months of the project.
The given information makes no distinction between the third, the fourth, and the fifth month. If it were possible to infer from this information a 30% probability for an incident in the fourth month, then it would be equally possible to infer a 30% probability for an incident in the third month and a 30% probability for an incident in the fifth month. And the only way for that situation to result in a 30% probability for an incident sometime in those three months is for the events "incident in month 3", "incident in month 4", and "incident in month 5" to be (not independent as some have suggested but) identical except for possible differences of probability zero.