The main problem with this questions is that the Sophists were not an actual school of thought. The Older Sophists (as they are properly called to distinguish them from later writers of the "Second Sophistic") is a term used to refer to several well-known intellectuals of classical Greece, most of whom earned their living by a combination of teaching and display orations. As a multiple choice question, this really assumes a model of scholarship on sophistic thought that has been called into question ever since the work of Grote in the mid-nineteenth century.
While the terms "cultural relativist", "sceptic", and "teacher of rhetoric" do apply to Protagoras and Gorgias, they are not entirely accurate for Prodicus. Protagoras' "homo-mensura" statement is considered and extreme statement of cultural relativism, as does his agnosticism about the gods. Gorgias' "On Non-Being" expresses extreme epistemological scepticism as well.
The answer your instructor wants here is A, because it contrasts a Platonic conception of unchanging reason with sophistic relativism and scepticism, but it does oversimplify both Plato and the Older Sophists.