Which one of the following engines will have heavier flywheel than the remaining ones?

The flywheel's weight is primarily determined by the engine's firing frequency, which is related to the number of power strokes per minute. Since the two-stroke diesel engine has the lowest number of power strokes per minute (750 r.p.m.), it will experience larger fluctuations in torque between power strokes, necessitating a heavier flywheel to smooth out these fluctuations.

The heavier flywheel is required for engines with lower firing frequencies (lower RPM) to help smooth out power delivery and reduce vibration. Since both Options C and D have the same RPM (750 r.p.m.), we need to consider the type of engine.

The two-stroke diesel engine tends to have more vibration compared to a four-stroke engine due to its firing frequency and fewer cycles to absorb the power strokes. Therefore, the two-stroke diesel engine (Option C) will likely require a heavier flywheel compared to the four-stroke diesel engine (Option D).

Hence, the correct answer is: Option C: 30 kW two-stroke diesel engine running at 750 r.p.m.