The word that best completes the sentence is "fanatical."
In this context, "fanatical" describes fans who are extremely enthusiastic and passionate about their team, often to an obsessive degree, which aligns with spending incredible amounts of time and money to follow them.
The other options do not fit the context as well:
- "fatuous" means silly or pointless and does not relate to the dedication of fans.
- "flagrant" means something that is glaringly bad or reprehensible and doesn’t relate to fan behavior.
- "feline" refers to cats and is unrelated to the topic of fans.
Therefore, the best choice is "fanatical."