In Order for Consent to Be Voluntary, Which Must Occur

In order for consent to be voluntary, which must occur?

A) The participant must sign a consent form.
B) The participant cannot be mentally incompetent.
C) The participant cannot be shamed, forced, or cajoled into participation.
D) The participant will not be paid (remunerated) because this would be coercive.
E) The researcher must confirm that the person signing the consent form truly understands what the research will involve.