Answer:
The correct option is;
D. Self-selected
Explanation:
Self-selected sampling also known as volunteer sampling is a sampling method in which participants are those that voluntarily respond to an advert or to the request of the researcher who ask for members of the population to take part in the study
The magazine in the question has put out a survey of which the magazine is asking readers to complete and the readers can then decide if they wish to participate in the study which is a form of self-selected sampling.