Sorry
That's Incorrect

Yes. Ethical reasons is correct, but in the context of this
question it is only partly correct. What are
the ethical reasons? For example, suppose
that you have presented a treatment to reduce aggressive
behavior in an emotionally disturbed child. With an AB
design we can see that during the baseline measurement
phase A (observation but no treatment) aggressive behavior)
is very high and that in the treatment phase B, aggressive
behavior is significantly less or non-occurring. Now, to
demonstrate that the manipulation was in fact the reason
for the change, we return the conditions to A and find that
the aggressive behavior is again high. It would be ethically
unacceptable to leave the child back at that baseline level.
Therefore, we insert another treatment B condition to bring
the aggression down again and leave the child at this more
acceptable level. Now why is this answer only
partly correct in the context of this multiple choice question?
Go over the other choices and you will, hopefully, see why.