Answer:
The best option is c. an ABAB experimental design.
Step-by-step explanation:
ABAB research design, also called withdrawal or reverse design, aims to observe if a treatment is being effective by exposing the subject to treatment, observing the results, then once again exposing to treatment, and confirming (or not) the results. The first A is the establishment of a steady baseline. The first B, then, is the introduction of the treatment. After the second baseline has been measured during B, the treatment is withdrawn during the second A. Again, having the baseline measured, the second B is the reintroduction of the treatment. That is precisely the method used with Fred to see how he responds to the reinforcement to speak.