Final answer:
The social worker should first discuss the parents' reasons for reading their 16-year-old daughter's email.
Step-by-step explanation:
When parents reveal during a family therapy session that they have been reading their 16-year-old daughter's email and are aware of her sexual activity, the social worker should first discuss the parents' reasons for reading her email (Option A). This allows the social worker to gather more information and understand the parents' perspective. By doing so, the social worker can work towards addressing any underlying issues, such as trust or communication, that may be contributing to the parents' invasion of their child's privacy. It is important to create an open and non-confrontational atmosphere where both the parents and the adolescent can express their thoughts and concerns.