Answer:
A privilege.
Step-by-step explanation:
A privilege can be defined as an advantage or prestige given to someone or a group of people or a societal group. The person(s) who have privilege have benefits of it more than the other person or the group. They can enjoy its advantages beyond than others.
The field trip that the teacher is talking about to take her students on is available only to those who scores more than 90% or higher in the class.
This field trip is a privilege to those students who score higher or equal to 90%.
So, the correct answer is 'a privilege'.