The correct answer is C) Fewer moving parts. A single-shaft gas turbine engine has a simpler design with fewer moving parts compared to a split-shaft gas turbine engine. The single-shaft design integrates the compressor, turbine, and load into a single rotating assembly, resulting in a more compact and mechanically less complex engine. This simplicity translates to fewer moving parts, which can be an advantage in terms of maintenance, reliability, and overall durability. The other options are incorrect because: A) Lower starting torque is not an inherent advantage of a single-shaft gas turbine engine. B) Reversibility is not a feature of a single-shaft gas turbine engine, as it is typically not designed for reversible operation. D) Fuel economy is not necessarily better in a single-shaft gas turbine engine compared to a split-shaft design, as it depends on the specific engine characteristics and operating conditions.