The correct answer to this open question is the following.
Although the question does not provide any options, we can say that the ethical standard that waiting to propose the study violates is "Informed Consent to Research."
In this case, Professor Hammond, first, had to inform the students about the study she was doing so the students were aware of this situation is class. So once she notified the class about her research, then ask them their consent to participate in the study. They probably had to sign a document that serves as an agreement of their participation. The students then would be aware of the details of the research and they would decide if they want to be part of the study. That is the ethical way to proceed.