The principle grants the right of private defence with one built in limit, it disappears whenever the person had time to have recourse to public authorities instead of taking matters into their own hands. So the first question is not whether X used reasonable force, it is whether that time to seek help existed.
X found out about the planned attack at 5 pm, a full six hours before it happened at 11 pm, and a police station was only a kilometre from his home. That combination of a long lead time and a nearby police station is precisely the scenario the principle's limit is written for, X plainly had the opportunity to have recourse to public authorities and chose not to take it.
Because that opportunity existed and went unused, the right of private defence never came into play for X or his family, regardless of how reasonable their later use of force may have looked in the moment. The failure to inform the police is the specific fact that decides the outcome here, not the fight itself.
So the correct answer is X and his family is liable as they have not informed the police.
Testing each option by asking what fact would need to be different exposes the issue with three of the four choices.
Since the facts already supply the very thing that defeats a private defence claim, unused time and an available police station, the option built on that failure is the one that fits without any adjustment.
Therefore, the correct answer is X and his family is liable as they have not informed the police.