ANS:A - heat rejection is lower.

In both the Otto and Diesel cycles, heat rejection occurs during the fourth process, which is the constant volume heat rejection process. This process involves the expulsion of waste heat from the working fluid to the surroundings, typically through the engine's cooling system. The efficiency difference between the Otto and Diesel cycles primarily arises from the differences in their combustion processes:

1. Otto Cycle: In the Otto cycle, combustion occurs at constant volume. This means that all the heat supplied during combustion is converted into work, resulting in higher efficiency.
2. Diesel Cycle: In the Diesel cycle, combustion occurs at constant pressure. While this allows for higher power output and better fuel economy in diesel engines, some heat is lost to the surroundings during the combustion process, leading to lower efficiency compared to the Otto cycle.