Final answer:
Brandi's train experience started as an emergent group and transitioned into a planned group as the commuters developed structured interactions and norms through regular social activities together.
Step-by-step explanation:
The grouping that best describes Brandi's train experience is d-They are an emergent group that became a planned group. Initially, Brandi and the other commuters did not have any structured interaction or established norms, making them an emergent group that formed organically due to their shared commuting times.
Over time, as they began to exchange greetings and participate in shared activities like a study club and karaoke outings, they transitioned into a planned group with more structured interactions and established norms.