I believe the answer is: b. Conduct a case study by interviewing the neighbor over the course of a year, but remember that anything learned about his altruistic behaviors might not be relevant to the rest of the population.
Case study is being done in order to find a development that occurs to a certain individual or groups within a certain period of time after giving them some sort of condition. Sorrentino could use this to measure which type of encouragement is beneficial and resulted in the largest amount of increase in people who adopts stray animal.