Answer: A) subjunctive.
Step-by-step explanation: The subjunctive is a verb mood that expresses a possibility and is often used for wished, imagined, desired, or possible (but not definite) states of being. In the given sentence the form of the underlined verb (participate) is subjunctive (corresponding to option A) because it is expressing the desire of the Student Council (represented by the verb "expects") the fact that everyone participates in Sunglasses Day, is a possibility and a desire, but it is not definite.