Explanation:

The correct answer is option 'B' which states that only the effects of strain ageing and orange peel effect are true in relation to yield point. Let's understand each of these effects in detail:

1. Luders bands:
Luders bands are wavy or serrated bands that appear on the surface of a material during plastic deformation. They are formed when the material undergoes localized plastic deformation before the onset of uniform plastic flow. Luders bands occur just above the yield point and are typically seen in low carbon steels. However, they are not directly related to the yield point itself.

2. Strain ageing:
Strain ageing is a phenomenon that occurs in some materials, especially low carbon steels, when they are subjected to plastic deformation and then exposed to elevated temperatures. It leads to an increase in the yield strength and a decrease in the ductility of the material. Strain ageing is caused by the diffusion of carbon atoms to dislocations, which results in the formation of carbides and reduces the mobility of dislocations. This phenomenon is true and directly related to the yield point.

3. Blue brittleness:
Blue brittleness is a phenomenon that occurs in certain types of steel, such as high carbon steel, when they are heated to a specific temperature range. At this temperature range, the steel becomes brittle and prone to cracking, even under minimal stress. However, blue brittleness is not directly related to the yield point and is not mentioned in the given options.

4. Orange peel effect:
The orange peel effect is a surface defect that can occur during the forming or stamping of sheet metal. It is characterized by a wavy or wrinkled appearance on the surface of the metal, resembling the texture of an orange peel. The orange peel effect is caused by non-uniform plastic deformation, which can be attributed to various factors such as material properties, tooling design, and process conditions. This effect is true and can be associated with the yielding behavior of the material.

To summarize, only the effects of strain ageing and orange peel effect are true in relation to the yield point. Luders bands and blue brittleness are not directly related to the yield point and are not mentioned in the given options. Therefore, the correct answer is option 'B' - 2 and 4.