I am not completely sure about the answer. But let's say that B is not correct because he felt very strongly about executing Charles and thought that was the only way to save the country. And D is also incorrect, because his son succeeded him as Lord Protector. I also think that A may be incorrect, because following the English Civil War, there was a time of Commonwealth of England, and then Cromwell was appointed Lord Protector, but he didn't do so himself, his fellow leaders invited him to do so.
So finally, in my opinion, the correct answer is C - because a great number was killed during the invasion of Ireland, he killed them all ruthlessly.
