What type of stress occurs during the operation of a mechanical design?

Dynamic stress is the correct choice because it refers to the stresses that occur in a mechanical design when it is subjected to changing loads, movements, or forces during operation. This type of stress arises in components of machines that experience varying forces over time, such as when machinery is in motion, including situations like the vibration of engines, rotating shafts, and impact loads.

Dynamic stress differs from static stress, which is constant and doesn't change over time. Elastic stress, on the other hand, refers to temporary stress that occurs when materials are deformed elastically, but does not directly address the dynamic application. Fatigue stress relates to the weakening of a material due to repeated cycles of stress, which can also be a significant factor in the performance of machine components, but it is ultimately a result of dynamic loading conditions, rather than a type of stress experienced during normal operation. Thus, dynamic stress captures the essence of operational conditions better than the other options.