Sternly, maybe could just be grim ( formiadable, harsh, uninviting, depressing, lack of humor, etc)
To be a good prince you have to put your people before anything
The word that could fit the gap is definitely a possessive pronoun such as their, used because it refers to the previous pronoun them
Answer:
d. by acknowledging that some people disagree with his position
Explanation:
Roberto addresses this new information "by acknowledging that some people disagree with his position"
Roberto cannot ignore the new source or call it foolish but can only acknowledge it and then finds better reasons to support his own claims above what the new information says.