Answer:
Truce.
Explanation:
A truce is an agreement between parties to stop fighting or any form of disagreement over an issue. In other words, when two or more parties agree to stop, though mostly temporary, their disagreement over a short period of time, it is called a truce.
In the given scenario, the narrator and her best friend Sol made the temporary agreement to be happy with each other's success even if that meant the other fails. This sportsmanship, where one is happy for the other's success even if one fails, shows the true nature of their friendship.
Thus, the correct answer is truce.