The correct answer is A. Moist loam.
Moist loam is a type of soil that is made up of a mixture of sand, silt, and clay. It is a good material for power shovels because it is not too hard or too soft. It is also a good material for growing crops.
Good common earth is a type of soil that is made up of a mixture of sand, silt, and clay. It is a good material for power shovels because it is not too hard or too soft. However, it is not as good as moist loam for growing crops.
Well blasted rock is a type of rock that has been blasted with explosives. It is a hard material that is difficult for power shovels to move.
Wet sticky clay is a type of clay that is wet and sticky. It is a difficult material for power shovels to move.
In conclusion, the output of power shovels for a fixed shovel size will be maximum for moist loam.