Introduction:
In a polyphase system, multiple phases of alternating current are used to transmit power. These phases are typically generated by generators or power sources and are used to distribute power to loads. To accurately measure the power in a polyphase system, wattmeters are used. The number of wattmeters required depends on the configuration of the system.

Explanation:
The number of wattmeters required to measure power in a polyphase system is equal to the number of wires. This is because each wire carries a different phase of the power, and a separate wattmeter is required to measure the power in each phase.

Reasoning:
Each wire in a polyphase system carries a different phase of the power. The power in each phase can be measured independently using a wattmeter. A wattmeter is a device that measures the power in an electric circuit by measuring the voltage and current and calculating the product of the two. In a polyphase system, each phase has a different voltage and current, and therefore requires a separate wattmeter to measure the power accurately.

Example:
Let's consider a three-phase system with three wires, A, B, and C. Each wire carries a different phase of the power, i.e., wire A carries phase A, wire B carries phase B, and wire C carries phase C. In this case, three wattmeters are required to measure the power accurately. One wattmeter is connected between wire A and wire B to measure the power in phase AB, another wattmeter is connected between wire B and wire C to measure the power in phase BC, and the third wattmeter is connected between wire C and wire A to measure the power in phase CA.

Conclusion:
In a polyphase system, the number of wattmeters required to measure power is equal to the number of wires. Each wire carries a different phase of the power, and a separate wattmeter is needed to measure the power accurately in each phase. Therefore, option A, "Number of wires," is the correct answer.