The sentence <em>No estén portándose mal en el salón de clases </em>is given in the imperative mood. We use this mood to tell someone to do something in a direct manner. Put another way, sentences in the imperative mood are commands. Perhaps the speaker of this sentence is a teacher that tells his/her students not to misbehave in classes.
I believe the correct answer is C because it is the only one that sounds iffy as it contains present and past tense. Answer choices A and B sound fine. Hope this helps!
