Answer:
B. Cross-Sectional
Step-by-step explanation:
A Cross-sectional study of design is a kind of design of observational study . In the cross-sectional study, an investigator measures or examines the outcome as well as the exposures in any study participants at the same time.
Cross-sectional design research includes using some different category of the people who differ in some variable of interest but they share few other characteristics, like educational background, socioeconomic status and ethnicity.
A Cross-sectional studies are mostly used by experimenters or researchers to study the developmental psychology in humans.
Thus in the context, the researcher using 50 participants to study the developmental changes in their choices to listen to music is a type of cross sectional research design.
Hence the correct option is (B).