What term describes someone who is overly preoccupied with something?

The term "obsessive" accurately describes someone who is overly preoccupied with something. It refers to a state of mind where a person becomes fixated on a particular thought, idea, or activity to such an extent that it dominates their attention and can impact their daily life. This obsessive nature often leads to repetitive behaviors or compulsive actions related to the object of their fixation.

In contrast, "recluse" refers to a person who avoids social interactions and prefers to live in seclusion, which does not necessarily imply preoccupation. "Paucity" denotes a scarcity or lack of something, and "mundane" refers to the ordinary or commonplace, neither of which convey the intense focus and fixation implied by being obsessive. Therefore, the choice of "obsessive" is the most fitting to describe the scenario of someone who is excessively fixated on something.