Advantages of Welded Joints over Riveted Joints:

1. Lighter and Less Costly Assembly:
- Welded joints eliminate the need for additional fasteners like rivets, bolts, or screws.
- This results in reduced weight and cost of the assembly.
- Welding requires fewer materials and fewer manufacturing steps compared to riveting.

2. Tight and Leak-Proof Assembly:
- Welded joints offer better sealing capabilities compared to riveted joints.
- The fusion between the base materials in welding ensures a continuous and solid connection.
- This eliminates gaps or spaces that could potentially allow leaks or the ingress of foreign substances.

3. No Stress Concentration Problem:
- Stress concentration occurs when there is a sudden change in the shape or cross-section of a component.
- Riveted joints have inherent stress concentration points at the rivet holes, where the load is concentrated.
- Welded joints, on the other hand, distribute the load more evenly across the welded area, minimizing stress concentration.

Explanation for Option 'C' (Thermal Distortion in Parts):

- Welding involves the application of heat to melt and fuse the base materials.
- This localized heating can cause thermal distortion in the parts being welded.
- Thermal distortion refers to the changes in shape or dimensions of the welded components due to the heat input during welding.
- The amount of distortion depends on factors such as the welding process, material properties, joint design, and fixturing.
- Distortion can lead to dimensional inaccuracies, misalignment, or warping of the welded assembly.
- In some cases, additional machining or corrective measures may be required to address the distortion.
- Riveted joints, on the other hand, do not involve the application of heat and therefore do not cause thermal distortion.
- This is why option 'C' is the correct answer, as it states that thermal distortion is not an advantage of welded joints over riveted joints.

In summary, while welded joints offer advantages such as lighter and less costly assembly, tight and leak-proof connections, and no stress concentration problems, they can introduce thermal distortion in parts being welded. Riveted joints, on the other hand, do not have this disadvantage as they do not involve the application of heat.