How Would a Professor Who Wants to Have His Students

How would a professor who wants to have his students provide data for a research study go about doing that, without involving coercion?

A) Mention that participation provides extra points.
B) Have a research assistant consent all participants and collect all data.
C) Offer extra points to the whole class if 50% of them act as participants.
D) Open the study to all students on campus and provide nonacademic incentives.