Answer:
D. A sample in which the subjects themselves decide whether to be included in the study.
Explanation:
Voluntary response sample: This is a sample made up of volunteers. These samples are mostly biased. For example, a person being called in a radio show poll might have powerful opinions over a topic in any direction. In a voluntary random sample, chooses themselves by answering a common appeal. These samples are unreliable because they are bias, hence the result is usually biased.