In this scenario, Susan's situation involves the potential violation of her rights based on her ethnic origin. To determine the appropriate area of law that applies, let's consider each option:

1. Constitutional law because constitutional rights were violated: This option is the most applicable. In the United States, the Constitution provides protection against discrimination based on race, ethnicity, or national origin. Specifically, the Equal Protection Clause of the 14th Amendment prohibits states from denying any person within their jurisdiction the equal protection of the laws. If Susan believes her rejection was due to her ethnic origin, this could be a violation of her constitutional rights, making constitutional law relevant to her case.
2. International law because Susan is an Asian-American: This option is not applicable. International law typically deals with the relationships between nations and is not directly concerned with individual cases of discrimination within a single country. Being Asian-American does not invoke international law in this context.
3. Constitutional law as it can be applied to any cases: This statement is too broad. While constitutional law does apply to many cases, it is specifically relevant here due to the potential violation of Susan's constitutional rights related to equal protection and non-discrimination.

Therefore, the correct answer is that constitutional law is applied in this case because Susan's constitutional rights may have been violated due to alleged discrimination based on her ethnic origin.