The word that best completes the sentence is "fanatical."
The context of the sentence refers to fans who are highly enthusiastic and devoted to the team, going to the extent of investing significant time and money in supporting them. "Fanatical" perfectly captures this notion of extreme enthusiasm and fervor, while the other options do not fit the context as well:
- Fatuous means silly or pointless.
- Feline refers to cats or characteristics of cats.
- Flagrant means something that is conspicuously and outrageously bad or offensive.
Therefore, "fanatical" is the most appropriate choice.