Final answer:
The email aims to highlight the traffic light timing issue at the school's Main Street entrance, causing delays and safety concerns. So the correct answer is Option C.
Step-by-step explanation:
The purpose of the email written by Joanna Mason to Principal Miller is to introduce a problem with the timing of traffic lights at the high school's main entrance on Main Street. Joanna explains that the traffic jams caused by the short green light at the intersection are leading to students being late and increasing the risk of accidents. She urges the principal to look into these issues to prevent a more serious accident.