What must be removed before taking out the piston?

The correct choice is the piston ring, as it must be removed before taking out the piston. The piston rings are designed to fit snugly within the cylinder and create a seal that prevents the combustion gases from escaping and oil from entering the combustion chamber. They are located in grooves on the piston and serve crucial functions such as sealing the cylinder, controlling oil consumption, and assisting in heat transfer.

Before the piston can be removed, the rings need to come off first, as they would obstruct the piston from sliding out of the cylinder. Removing the rings allows for a clear pathway, ensuring that the piston can be extracted smoothly without causing damage to either the piston or the cylinder wall.

The other options, such as the piston cap, piston rod, or piston sleeve, may be components related to the piston assembly but removing them is not necessary in the direct process of piston removal. The immediate concern when dealing with piston extraction is to properly handle the piston rings first to avoid any complications during the procedure.