The up-and-down motion of the piston is converted to rotary motion by the:

A. connecting rod and the crankshaft.
B. push rods and the rocker arms.
C. cylinder head and the valves.
D. bearings and the piston rings.

Answer :

Answer is A) The connecting rod and crankshaft

The connecting rod is attached to a piston, which is pushed up and down due to pressure created by a heat-generating reaction in its chamber. As the piston is moved by the this pressure, its connecting rod, attached to the crank shaft, rotates the crank shaft. As the crankshaft completes a rotation, the piston is compressed to its original spot, again due to the attached connecting rod, and the cycle repeats.
It is definitely connecting rod and crankshaft. Im studying this on PF right now, and I just looked it up in the study guide.