What is the term used when someone feels they are treated unfairly by an employer?

The correct term used to describe feelings of being treated unfairly by an employer is "unfairly." This word directly relates to the notion of injustice in treatment or actions. When someone feels they have been dealt with in a way that does not reflect equity or fairness, they might express that they have been treated unfairly. This term captures the essence of the emotional or subjective response someone might have to their situation in the workplace.

The other terms provided have different connotations. "Unjust" generally refers to actions or situations that are not justifiable, often used in a broader societal context rather than strictly within interpersonal relationships. "Unduly" implies something excessive or unwarranted, which can describe a situation but does not directly express the feeling of unfair treatment. "Unreasonably" suggests a lack of reason or logic but doesn’t specifically denote unfairness in the way that "unfairly" does. Thus, "unfairly" is the most appropriate choice for conveying that someone feels wronged or mistreated in their work environment.