The most appropriate basis for the apportionment of power is horse power of machines. This is because the amount of power a machine has is directly proportional to the amount of work it can do. Therefore, apportioning power based on horse power will ensure that the most efficient machines are used, and that the work is divided fairly among the machines.
Option A, number of labor, is not a good basis for apportionment of power because it does not take into account the efficiency of the workers. Some workers may be more efficient than others, and some machines may be more efficient than others. Therefore, apportioning power based on the number of workers would not necessarily result in the most efficient use of power.
Option B, number of output, is also not a good basis for apportionment of power because it does not take into account the quality of the output. Some machines may produce higher quality output than others. Therefore, apportioning power based on the number of output would not necessarily result in the best quality output.
Option C, number of machines, is not a good basis for apportionment of power because it does not take into account the size or capacity of the machines. Some machines may be larger or have a higher capacity than others. Therefore, apportioning power based on the number of machines would not necessarily result in the most efficient use of power.
Therefore, the most appropriate basis for apportionment of power is horse power of machines.