Approximate estimation of power demand can be made by

The correct answer is: D. All of the above

Load survey method, statistical methods, and mathematical methods are all used to estimate power demand.

• Load survey method is a direct method of estimating power demand. It involves measuring the actual power consumption of a load over a period of time. This method is accurate, but it can be expensive and time-consuming.
• Statistical methods are indirect methods of estimating power demand. They use historical data on power consumption to predict future power demand. This method is less accurate than the load survey method, but it is less expensive and time-consuming.
• Mathematical methods are also indirect methods of estimating power demand. They use mathematical models to predict future power demand. This method is less accurate than the load survey method and statistical methods, but it is less expensive and time-consuming.

The best method to use for estimating power demand depends on the specific situation. If accuracy is important, then the load survey method is the best option. If cost and time are important, then statistical or mathematical methods are better options.