Final answer:
The flywheel is mounted on the rear of the main journals and the block main bore of an engine to smooth out engine vibrations caused by piston firings.
Step-by-step explanation:
A) Flywheel
The correct answer is A) Flywheel. Nearly all conventional piston engines have flywheels to smooth out engine vibrations caused by the thrust of individual piston firings. The flywheel acts as a rotating mass that stores and releases energy, helping to balance out any uneven forces produced by the firing of the pistons. By absorbing and dissipating these vibrations, the flywheel helps to stabilize the engine's operation and improve overall smoothness.