Explanation:

- A Wheatstone bridge is a fundamental electrical circuit used to measure unknown electrical resistances by balancing two legs of a bridge circuit.

- The standard configuration of a Wheatstone bridge includes four resistors.

- Two resistors are typically known, one is the unknown resistor to be measured, and the fourth resistor is variable to balance the bridge.

- Option:1, Three resistors would not properly fulfill the bridge's requirements, as at least four are needed for balancing.

- Option:2, Six resistors exceed the basic design of the traditional Wheatstone bridge.

- Option:3, Five is also incorrect for the traditional Wheatstone bridge.

- Option:4, Four is indeed the correct number of resistors for a Wheatstone bridge.