Electric resistance welding is a welding process used to join metal parts together using the heat generated by passing an electric current through the materials. In this process, the voltage requirement plays a crucial role in determining the quality and effectiveness of the weld.

The correct answer is option 'A', which states that the voltage requirement for electric resistance welding is 6 to 10 volts.

Explanation:
1. Electric Resistance Welding: Electric resistance welding is a process that involves the application of pressure and heat to join metal parts together. It is commonly used in the manufacturing industry for various applications, including automotive, construction, and pipe welding.

2. Voltage Requirement: The voltage requirement for electric resistance welding depends on several factors, including the type and thickness of the metal being welded, the desired weld quality, and the specific welding equipment being used.

3. Optimal Voltage Range: The optimal voltage range for electric resistance welding is typically between 6 and 10 volts. This voltage range is suitable for most common welding applications and provides a balance between heat generation and control.

4. Factors Affecting Voltage Requirement: The voltage requirement can vary based on the type of metal being welded. For example, thicker metals may require higher voltages to generate enough heat for proper welding. Similarly, the desired weld quality, such as penetration depth and fusion, can also influence the voltage requirement.

5. Welding Equipment: The welding equipment used in electric resistance welding includes a power supply, electrodes, and clamps. The power supply is responsible for providing the necessary voltage and current to generate the required heat for welding. The electrodes and clamps are used to apply pressure and hold the metal parts together during the welding process.

6. Importance of Voltage Control: Controlling the voltage is essential in electric resistance welding to ensure consistent and reliable welds. Too low of a voltage may result in inadequate heat generation, leading to weak or incomplete welds. On the other hand, excessive voltage can cause overheating and damage to the metal parts.

In conclusion, the voltage requirement for electric resistance welding is typically in the range of 6 to 10 volts. However, it is important to note that this range can vary depending on factors such as the type and thickness of the metal being welded and the desired weld quality. Proper voltage control is crucial to achieve high-quality welds in electric resistance welding.