The correct answer is C) An unusually long ocean swell. The first sign of a well-developed tropical cyclone is the presence of an unusually long ocean swell. This is because the cyclone's strong winds generate large, long-period waves that travel outward from the storm center, often reaching land before the cyclone itself. These swells can provide an early warning of an approaching tropical system, even before other signs such as increased thunderstorm activity or higher humidity become evident. The other options, while potentially associated with tropical cyclones, are not the earliest or most definitive signs of a well-developed storm.