Three active Sync agents is the correct choice.

Three active Sync agents provides a simple majority for leader election and quorum which keeps the synchronization service available when one agent fails. Running three active agents gives redundancy and it allows the system to make progress with a single failure while avoiding a single point of failure.

Two active Sync agents is incorrect because two agents cannot form a stable majority if one fails and they are prone to split decisions without an additional voter.

Four active Sync agents is incorrect because an even number of agents can lead to tied votes and it is not the recommended configuration for avoiding split decisions. An odd number of agents is normally preferred for robust quorum behavior.

One active Sync agent is incorrect because a single agent is a single point of failure and it does not provide any redundancy for high availability.

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