What category is defined as any weapon that may expel a projectile by air or explosives?

The category defined as any weapon that may expel a projectile by air or explosives is accurately represented by the term "gun." This encompasses a wide variety of weapons, such as handguns and rifles, that function by using air pressure or explosive force to launch a projectile. The broad definition aligns with legal and technical descriptions, which categorize guns under firearms, as they are engineered to discharge projectiles through a barrel, utilizing various propellant mechanisms.

The other choices, though related, do not encompass the full range of what "gun" implies. A knife is a cutting instrument and does not expel projectiles. While a rifle is a type of gun, it is a more specific classification under the broader category of guns. The term "firearm" generally includes all guns that are capable of discharging projectiles and may be used interchangeably with "gun" in some contexts; however, "gun" is the more precise term for the broad definition provided in the question.