The piece of evidence that would be most likely to weaken the argument for kin selection driving altruistic behavior in groups of related individuals is:
Data showing that only distantly related individuals are most likely to perform altruistic behaviors in order to protect each other.
This evidence suggests that altruistic behaviors are observed among individuals who are not closely related, contrary to the expectation of kin selection, which predicts that altruism should be more common among those who share a higher percentage of DNA. If altruism arises more frequently among distantly related individuals, it implies that factors other than genetic relatedness are influencing altruistic behavior.