If someone is sitting pensively, how are they sitting?

When someone is described as sitting pensively, it suggests that they are lost in thought or deeply reflecting on something. This posture typically conveys a serious or contemplative state of mind rather than a relaxed or hurried demeanor. A thoughtful position often includes a furrowed brow or a distant gaze, indicating that the individual is engaged in contemplation or serious reflection about a matter, emphasizing the mental engagement that accompanies the physical posture.

In contrast, the other choices represent different states of being. Restfully implies a more relaxed and leisurely approach, hastily suggests urgency and quickness, while carelessly connotes a lack of attention or regard for the situation. None of these alternatives capture the introspective quality that accompanies a pensive sitting posture, hence thoughtfully is the most appropriate description in this context.