Scientist: "Recent research in neuropsychology

Premise 1: In one study, a social scientist observed instances of laughter in diverse settings, and found that 80 to 90 percent occurred after statements not intended to have humor, such as "talk to you later" and "I know:"

Conclusion: laughter is less a response to humor than a kind of involuntary social signal.

To understand the logic of the question, you need to know what "social signal" means.

The conclusion means people are more likely to laugh when they got social signals than when they response to humor.

Social signal means people assume the actions of others in an attempt to reflect correct behavior for a given situation. On the other hand, it doesn't matter whether one says <"talk to you later" or "I know"> or not (express the social signal by saying a specific statement), other people still understand the situation and reflect by laughing. So to make a conclusion valid, we need to assume people do not have to wait until others state something to laugh. If there is a social signal, people will react by laughing.

C says exactly the same, so it's the correct assumption.

Hope it helps.