Final answer:
The factor that significantly decreased conformity in Asch's line studies was participants having at least one ally who was willing to resist conformity. Therefore, the correct option is D.
Step-by-step explanation:
The factor that significantly DECREASED the likelihood of conformity in Solomon Asch's famous "line studies" was d) Participants had at least one ally who was willing to resist conformity. In Asch's experiments, when a participant had a single ally who dissented from the group's incorrect answer, the subject conformed only a quarter as often to the group pressure. Having an ally who was also willing to resist conformity provided support and decreased the participant's need to conform to the group.