In each diagram, ∆abc has been transformed to yield ∆a'b'c'. which transformation could not be achieved by rotation alone?

The fourth diagram or the penultimate diagram is the one transformation among the following choices given in the question that could not be achieved by rotation alone. I hope that this is the answer that you were looking for and it has actually come to your great help. It is very important to analyze each diagram properly before getting to the answer.