What does the referee do if he inadvertently sounds the whistle during play?

When a referee inadvertently sounds the whistle during active play, the appropriate course of action is to award a dropped ball to the last team that had possession of the ball. This procedure is in place because the whistle interruption has halted the natural flow of the game, and a dropped ball helps to restart the match fairly.

In the context of the game, it's crucial for maintaining the integrity of play that the team that was in possession is returned to the same situation as closely as possible to how it was just before the whistle was blown. The dropped ball allows for a neutral restart without favoring either team, which is in alignment with the principles of fair play in soccer.

The other options, while they may seem plausible in different contexts, do not apply to this specific situation. Continuing the game would disregard the interruption caused by the whistle, and awarding a free kick would unfairly penalize one team without just cause. Also, permitting a substitute to enter does not relate to the immediate circumstance of the whistle being inadvertently blown during play.