Final answer:
The word that best fits the blank to describe the sports team's supporters who knew every player and cheered at all the games is 'ardent'. This term illustrates their passionate and dedicated nature.
Step-by-step explanation:
The sports team had very ardent supporters who knew every player and cheered at all the games.
The adjective needed to fill the blank pertains to the supporters' enthusiasm and dedication towards the sports team. Words like 'ardent', 'devoted', or 'passionate' are appropriate as they reflect the high level of support and enthusiasm these fans would exhibit during the games. They are the kind of supporters who are deeply committed, showing up for every game, knowing every detail about the players and the team, and providing consistent, vociferous support.