I don't disagree that changing the standard accepted response to behavior was appropriate. But conduct is a valid translation as well. Conduct is listed as a translation on Spanishdict.com.
Additionally, if you read the synonym study of behavior compared to conduct you would probably see many contexts (if not most) under which conduct would be better, especially since this sounds like a statement of a sociologist or cultural anthropologist.
—Synonym Study 1. Behavior, conduct, deportment, comportment refer to one's actions before or toward others, especially on a particular occasion. Behavior refers to actions usually measured by commonly accepted standards: His behavior at the party was childish. Conduct refers to actions viewed collectively, especially as measured by an ideal standard: Conduct is judged according to principles of ethics. Deportment is behavior related to a code or to an arbitrary standard: Deportment is guided by rules of etiquette. The teacher gave Susan a mark of B in deportment. Comportment is behavior as viewed from the standpoint of one's management of one's own actions: His comportment was marked by a quiet assurance.
If you read through the comment section you will find this same question asked and answered many times. The difference is grammatical. The subject of your sentence is this but the subject of their sentence is conduct. In context less sentences it seems like an irrelevant consideration, but in real life often one will be appropriate when the other is not. Case in point. Someone is looking for their red shoes and you are helping. You see something at the back of the closet that is red. Are those your red shoes? The answer comes No, that is my red purse. You spot a pair of shoes. Are those your red shoes? No, those shoes are pink. It would make no sense if you reversed them. You would never say that purse is red in answer to the first question, and you would be less likely to say those are pink shoes, although possible, to the second. There are situations where what the subject is effects the tone or matches different situations. That is why Duo wants you to translate using the correct construction
Very subtle difference. In the Spanish sentence you are describing the conduct. In your sentence you are defining "this". It doesn't make any real difference, but your sentence would be Eso es conducto universal. The sentence have different subjects and that is something that should be noted as it sometimes can effect a translation more.
I am never sure when the questions in these sections are actual questions and when they are ironic statements on life. But assuming that you might actually be asking, I will answer. When people talk about universal behavior they are saying that all people in all cultures, countries etc have some way of doing it. In some discussions it might actually cross species lines to cover behavior of all animals. Self preservation behavior, for example, is universal
Common is what you say about what you see around you. But universal behavior is saying that the behavior is part of the essence of being human. Every single person does it. That's the difference between the meaning of the two sentences. Of course the more specific reason the answer is universal and not general is that the Spanish word for universal is universal and the Spanish word for general is general. So although you really are barely translating, in order to complete the exercise you would never translate one for the other because they never mean the same thing.