Spot welding is a welding process in which two pieces to be joined are overlapped and placed between two electrodes. The correct answer to the given question is option 'D', spot welding.

Spot welding is widely used in various industries, including automotive, aerospace, and construction, due to its efficiency and effectiveness in joining metal components. It is a type of resistance welding, where heat is generated by the resistance to electric current flow at the point of contact between the two pieces being joined.

Below are the key points explaining spot welding in detail:

1. Spot welding process:
- In spot welding, two metal pieces are overlapped and placed between two electrodes, typically made of copper alloy.
- The electrodes apply pressure to hold the pieces together firmly.
- An electric current is then passed through the electrodes, creating resistance heating at the contact points.
- The heat generated by resistance causes the metal to melt, and when the current is stopped, the molten metal solidifies, forming a weld nugget.

2. Advantages of spot welding:
- High production speed: Spot welding is a fast process, making it suitable for high-volume production lines.
- Cost-effective: Spot welding requires minimal equipment and consumables, resulting in lower production costs.
- Strong and durable joints: Spot welding creates welds with high strength and integrity, ensuring the durability of the joined components.
- No additional materials: Spot welding does not require additional filler materials, as the heat generated by resistance melts the metal at the joint.

3. Applications of spot welding:
- Automotive industry: Spot welding is extensively used in the automotive industry to join sheet metal components, such as body panels, frames, and exhaust systems.
- Electrical industry: Spot welding is used to join electrical contacts, connectors, and components in appliances and electronic devices.
- Aerospace industry: Spot welding is utilized in the assembly of aircraft structures, fuel tanks, and engine components.
- Construction industry: Spot welding is employed in the fabrication of steel structures, such as bridges, buildings, and pipelines.

In conclusion, spot welding is a welding process in which two pieces to be joined are overlapped and placed between two electrodes. It is widely used in various industries due to its efficiency, speed, and strong weld joints.