Psychologist: The obligation to express gratitude cannot be fulfilled anonymously.
Conclusion
The obligation to express gratitude cannot be fulfilled anonymously.
Intermediate Conclusion (Thus)
The important social function of positively reinforcing helpful behaviors can be served only if the helping person knows the source of the gratitude.
Evidence
Human psychology is driven primarily by personal interaction.
Evaluate
Whenever we do Main Conclusion or Role (but only for these two question types), we prime our brain before reading the paragraph with the reminder that LSAC usually puts the Main Conclusion earlier than at least some of the Supporting ideas.
We listen specifically for "1st Sentence Conclusions" and "Rebuttal Conclusions".
Does this first sentence sound like the author expressing an opinion?
Yeah, sure. She's saying That sounds like an opinion.
Does she go on to try to convince us that we should let people know that it's us who are thanking them?
She does. She says that human psychology is still driven by personal interaction (the opposite of anonymous interaction). The social value of expressing thanks can be served only if the thankee knows who the thanker is.
Since the first sentence was an opinion and then the rest of the paragraph supports that opinion, we know we (once again) have a First Sentence Conclusion on Main Conclusion / Role.
Goal
Now we look at which claim the question stem is asking about and see they're asking about the first sentence.
So let's look for an answer that says or means Main Conclusion.