I am not sure I understand your multiple choices for answers, but the correct words to put into your sentence are: 'explains', and 'is'. Because the word 'either' has been used, that changes the the word after lantern from multiple to singular.
Well one reason why they struggled was because the priest had higher authority than the king himself did but I don't really know why or how it lead to the crusades in the holy land. I'm sorry I couldn't help you fully but I tried.
