Answer:
a. voluntary participation.
Explanation:
When doing research, one of the main characteristics a study should have is that participants cannot be forced to participate in the studio and they have to have a voluntary participation, otherwise, the results that the researcher gets might be biased because they don't actually represent a significant sample from the population.
In this example, Professor Smith wants to track the dating behaviors of college students and she decides to use her introductory class. She explains her research to the students and then they all complete her survey. They are not given the opportunity to decide whether they want to participate or not, therefore, her research most clearly presents a problem with voluntary participation.