The correct answer is most.
The word “most” is used to indicate that someone or something is greater than all others in a particular group or category. In this case, the speaker is saying that the person they are talking about is the person who is most to blame for the situation.
The other options are not as accurate. The word “mostly” means “to a great extent” or “for the most part.” This does not necessarily mean that the person is the most to blame, only that they are to blame to a great extent. The word “sure” means “certain” or “confident.” This does not necessarily mean that the person is to blame, only that the speaker is certain that they are to blame. The word “bound” means “obliged” or “required.” This does not necessarily mean that the person is to blame, only that they are obliged to take responsibility for the situation.