Answer:
The ansswer is a. case study.
Step-by-step explanation:
A case study is a research method that consists in analysing a single case, person or event. The information gathered can then be generalised in order to predict future events, or to understand current situations.
The greatest advantage of case studies is that they are usually inexpensive and provide high validity. However, it's possible that the studied person or group doesn't reflect the characteristics of a larger population.