Answer:
Dr. Samuelson is conducting a B. Cross-sequential study.
Explanation:
A Cross-sequential study is a mixture of the longitudinal and cross-sectional study.
Let's break them both down:
Thus, a cross-sequential study observes its various types of subjects through a long period of time.
It can be seen in this example: Dr. Samuelson's plan is to follow various subjects of different ages over 10 years in order to observe their use of social media and relationships.