Answer:
The principle violated by the choice of participants is: justice.
Step-by-step explanation:
The Belmont Report has three core principles concerning ethical guidelines for researches that have human subjects. They are: respect for persons, beneficence, and justice.
The principle of justice is usually violated when it comes to the choice of participants. According to this principle, the burdens and benefits of the research should be distributed. That means it is unfair for a certain group to bear the cost of the research for another group to benefit from it. Notice that, in professor Kwan's research, the group of participants consists of Hispanic children from low-income families. While this group carries the burden of being subjected to tests and studies, other groups from wealthier families and different ethnicity may very well benefit from the results. That violates the principle of justice.