The correct answer is D) problematic because users often are aware of and do not give consent for their participation.
The use of social media to do psychological research is problematic because users often are aware of and do not give consent for their participation.
Social media is not the best place and way to conduct formal and serious psychological research. Social media has other uses, mostly entertainment and distraction.
To do serious research, psychologists have to form groups with certain characteristics, depending on the purpose of the research. Sometimes, people have to be in the right kind of environment to help get better results.