The word that best completes the sentence is "fanatical."
Here's why:
- "Fanatical" describes someone who is extremely enthusiastic or devoted to a cause, often to an excessive degree, which fits the context of fans spending incredible amounts of time and money following their team.
- "Flagrant" refers to something that is shockingly noticeable or evident, often in a negative context, which doesn't align with the sentence's meaning.
- "Feline" relates to cats and has no relevance to fans or their behavior.
- "Fatuous" means silly or foolish, which doesn't capture the intense devotion implied in the sentence.
Therefore, "fanatical" is the most appropriate choice.