Final answer:
Whitney can provide the best reason to support joining a teacher's union by emphasizing that it allows members a voice on policies.
Step-by-step explanation:
The best reason that Whitney can provide to support joining a teacher's union is that it allows members a voice on policies.
Joining a union gives teachers the opportunity to participate in decision-making processes and have input on important policies that affect their work environment.
For example, union members may have the chance to vote on issues such as class sizes, curriculum changes, or disciplinary procedures, ensuring that their opinions are heard and considered.