The correct answer is C. antagonist.
An antagonist is a person who opposes or fights against someone or something. In the context of the question, the opponent is someone who is competing against you. It is important to never underestimate your opponent, regardless of how weak they may seem. This is because they may have hidden strengths or abilities that you are not aware of. Additionally, underestimating your opponent can lead to complacency, which can cost you the competition.
The other options are incorrect because they do not accurately describe the relationship between you and your opponent. A friend is someone who you like and trust. A rival is someone who is competing against you for the same thing. An ally is someone who is on your side and helps you in your fight.
It is important to remember that your opponent is not your enemy. They are simply someone who is trying to achieve the same goal as you. It is possible to have a respectful and professional relationship with your opponent, even if you are competing against them.