The underlined phrase <em>in the pool </em>modifies, or refers to the word A. chlorine.
What is in the pool water? The chlorine.
The correct answer should be C. Brittany
That is because her being late is something that can easily change and wouldn't really affect her life that much. She is rational about her being late. Hassan is close to becoming an alcoholic and would be hysterical without it meaning that his decision to drink is not rational.