What type of interaction is vital in the definition of a group?

The correct answer is social. Social interactions are fundamental to the definition of a group because they involve the meaningful connections and relationships among individuals who come together for a common purpose or interest. When members of a group interact socially, they establish communication, share experiences, and create bonds that contribute to group cohesion and identity. This social aspect is what distinguishes a group from a mere collection of individuals. While collaboration can be an essential component of some groups, it's the broader social interaction that is vital in forming a cohesive unit. Similarly, informed and conflictual interactions may occur within groups, but they do not capture the essence of what defines a group as fundamentally as social interaction does.