Because of the design type, blowback firearms are always:

The correct choice highlights a key characteristic of blowback firearms. Blowback is a mechanism in semi-automatic and automatic firearms where the force of the fired cartridge pushes the bolt or slide backward. This type of action usually utilizes the energy generated from the combustion of the cartridge and relies on the weight of the bolt or slide to help cycle the action.

Blowback firearms are typically designed to be magazine-fed because the ammunition is housed in a detachable or fixed magazine that feeds rounds into the chamber. This design is efficient and allows for quick reloading and operation. In semi-automatic pistols, blowback operation is common, and these firearms typically utilize a single stack or double stack magazine for ammunition.

While there are firearms that use gas operation and recoil operation mechanisms, those systems have different design functionalities than blowback. Gas-operated firearms, for example, use gas from the fired cartridge to cycle the action, leading to different operational dynamics. Likewise, recoil-operated mechanisms rely on the motion and energy created from recoil and may not correspond specifically to the characteristics of blowback designs. The mention of single action refers to the method of firing—indicating how the trigger mechanism is designed—but it does not specifically connect to the blowback operation itself.