Participants sometimes drop out of a longitudinal study. what term is used to describe this situation?

The correct answer is attrition. 

Attrition refers to the phenomenon in which people tend to drop out of psychological studies and experiments before the study is completed. Attrition negatively affects the validity of a study, because the sample size decreases and there is no longer sufficient data that the study can collect. Attrition occurs for many reasons, including: unforeseen circumstances, participants changing their mind about participating in thei study, moving to a different location, etc.