In the design of rivets and bolts for steel structures, the effect of wind or earthquake load is taken into consideration to ensure the safety and stability of the structure. These loads can create additional stresses on the connections, and therefore it is important to account for them in the design process.

When considering wind or earthquake load, it is common practice to increase the permissible stresses in rivets and bolts. This is done to provide a safety margin and to account for the uncertainties and variability in the actual loads experienced by the structure. The percentage by which the permissible stresses may be exceeded is typically specified in design codes and standards.

The correct answer to the question is option 'B', which states that the permissible stresses may be exceeded by 25%. This means that the design calculations should be based on a stress level that is 25% higher than the allowable stress for the material. This additional allowance ensures that the connections can withstand the increased loads and provides a factor of safety.

By exceeding the permissible stresses by 25%, the design accounts for the uncertainties and variability in the actual loads. It also ensures that the connections have sufficient strength to resist the wind or earthquake forces without experiencing failure or excessive deformation.

It is worth noting that the specific percentage by which the permissible stresses may be exceeded can vary depending on the design code or standard being used. Different codes may have different requirements and factors of safety. Therefore, it is important to refer to the applicable design code or standard to determine the exact percentage to be used in the design calculations.

In conclusion, when considering the effect of wind or earthquake load in the design of rivets and bolts for steel structures, the permissible stresses may be exceeded by 25%. This provides a safety margin and accounts for the uncertainties and variability in the actual loads experienced by the structure.