When the development of the same subjects is studied over a period of time, the study is called a: A. cross-sectional study. B. life history study. C. longitudinal study. D. sequential study.

Each option provided refers to a specific type of study. Let's briefly define each term: A. Cross-sectional study: A study that involves taking a snapshot of a group of subjects at a single point in time to understand a particular aspect. B. Life history study: A research method that examines an individual's entire life by collecting information on their personal experiences, relationships, and significant events. C. Longitudinal study: A study that focuses on observing the same subjects over a period of time to understand changes or development related to specific aspects. D. Sequential study: A combination of cross-sectional and longitudinal studies, in which researchers study different groups of subjects at varying time points to examine developmental trends.

According to the exercise, we are looking for a term of a study that focuses on the development of the same subjects over a period of time. Based on the definitions we explained in the step 1, the correct term is the "longitudinal study" (option C).