wheel and axle system, as found in vehicles
"Mechanical device" is too broad a term to accurately identify the best source of power for the least amount of effort. A conveyor belt does not require external effort, but is instead purely mechanical. Pulley systems require greater effort on the part of the user than a wheel and axle system. For this reason, wheel and axle systems are the greatest source of power for the least amount of effort.
B A pulley system, as might be found in warehouses and shipyards
C A conveyor belt, as those found in factories
D Any form of mechanical device