The correct answer is: A. to
The word “heir” means “a person who inherits property or a title from someone who has died.” In this case, the throne is the property that is being inherited. The word “to” is used to indicate that the heir is the person who is entitled to inherit the throne.
The other options are incorrect because they do not make sense in the context of the sentence. The word “with” would imply that the heir is someone who is associated with the throne, but not necessarily someone who is entitled to inherit it. The word “him” would imply that the heir is a specific person, but the sentence does not provide any information about who the heir is. The word “over” would imply that the heir is someone who has control over the throne, but the sentence does not indicate that the heir has any power or authority.