In the Remington model 870, the _______ is located inside the loading floor on the right and left and houses shells in the magazine.

The correct answer is found in the designation of the component responsible for holding shells in the magazine of a Remington model 870. The shell stop is a critical part that ensures that cartridges are held securely in the magazine until the action cycles and they are fed into the chamber for firing.

The shell stop's location inside the loading floor on both the right and left sides allows it to effectively manage the flow of ammunition as it interacts with the magazine tube. When the action is cycled, the shell stop releases a shell into the chamber while preventing additional shells from moving forward until it is necessary to load another round. This precise mechanism is essential for the proper function and reliability of the firearm.

Other components mentioned, such as the feed ramp, action bar, and magazine tube, serve different purposes within the firearm. The feed ramp is responsible for guiding the shell smoothly into the chamber, the action bar connects various operational mechanisms during firing, and the magazine tube houses the reserve ammunition for subsequent shots, but these components do not function as the mechanism to hold shells in place within the loading floor. Each component plays a unique role, but the shell stop specifically addresses the task described in the question.