Final answer:
Mail panel surveys can be used for longitudinal research, enabling the study of subjects over time, but they may suffer from low response rates and potential biases.
Step-by-step explanation:
The statement that is true of mail panel surveys is B. They can be used for longitudinal research. Mail panel surveys allow researchers to study the same subjects over a period, which helps in observing changes and trends. These surveys often result in low response rates and the sample may not be completely representative of the target population, leading to biased outcomes. Additionally, while the panel can be tested before a survey, mail panel surveys are not person-administered as they rely on the recipients to fill out and return the survey forms themselves.