Which rifle is characterized as a lightweight, air-cooled, gas-operated, magazine-fed shoulder weapon?

The description provided fits best with the M16 rifle, which is indeed characterized as a lightweight, air-cooled, gas-operated, magazine-fed shoulder weapon. The M16 represents a significant evolution in military firearms, designed for ease of use, reduced weight, and higher rates of fire, making it a staple in various military operations.

The M14A1, while a capable shoulder weapon, is heavier and does not possess the same level of gas operation efficiency or lightweight characteristics as the M16. The M4 Carbine is a derivative of the M16 and shares many features, including being lightweight and gas-operated, but it is shorter in length and designed for more specialized combat roles. The AR-15 is a civilian variant and shares similar characteristics with the M16 but is not classed as a military weapon.

In summary, the M16 is the correct choice because it accurately fits the criteria of being lightweight, air-cooled, gas-operated, and magazine-fed.