The answer to this is C. Benvolio. It was Benvolio who told Romeo that he needs to go to the party just to compare the other women to Rosaline whom Romeo loved. Benvolio’s goal is to show Romeo that the other women are so beautiful that Rosaline would pale in comparison. This was the time when Romeo did go and he saw Juliet for the first time.
It is safe to say that he forgot all about Rosaline after he saw Juliet who was just 13 years old at that time. Romeo and Juliet were both young when they met. This explains why their emotions are so strong and they made decisions without thinking too much of the consequences.