The correct answer is B. Value of Machines.
The value of machines is the most suitable basis for apportioning insurance of machines because it is the most accurate way to determine the risk involved. The value of the machines will be directly proportional to the cost of replacing them if they are damaged or destroyed.
The other options are not as accurate as the value of machines. Floor area is not a good basis for apportioning insurance because it does not take into account the value of the machines in the area. The number of workers is also not a good basis for apportioning insurance because it does not take into account the value of the machines that the workers use.
Therefore, the value of machines is the most suitable basis for apportioning insurance of machines.