The word that best completes the sentence is fanatical.
The sentence discusses fans who are extremely devoted and willing to invest a lot of time and money following the team. "Fanatical" describes someone who is excessively enthusiastic or passionate, making it the appropriate choice for this context.
The other options do not fit the context:
- Flagrant means something that is shocking or obviously bad.
- Feline relates to cats or characteristics of cats.
- Fatuous means silly and pointless.
Therefore, the correct choice is fanatical.