Answer:
Option 3 is right.
Explanation:
Given facts are:
A study of college students recruited participants by sending an email and asking them to take an online survey.
WE have to find out Is the sample a voluntary response.
No, the study sent the email to the entire population. -- Wrong since only selected were sent.
No, the respondents had no choice in whether to respond to the email.--Wrong because no action can be taken if not replied
Yes, the respondents themselves decide whether to participate in the survey.--Right because when they respond the mail, they show their willingness voluntarily to participate in the survey.
Yes, the study contacted only those students who wanted to be contacted.--Wrong since some may not reply at all.