Question: Heckler & Koch HK416 is a rifle largely based on what variant of the M16A2?
Context: The Heckler & Koch G11 is a non-production prototype assault rifle developed during the late 1960s, 1970s and 1980s by "Gesellschaft für Hülsenlose Gewehrsysteme" (GSHG) (German for "Corporation for Caseless Rifle Systems"), a conglomeration of companies headed by firearm manufacturer Heckler & Koch (mechanical engineering and weapon design), Dynamit Nobel (propellant composition and projectile design), and Hensoldt Wetzlar (target identification and optic systems).  The rifle is noted for its use of caseless ammunition. The M27 Infantry Automatic Rifle (IAR) is a lightweight, magazine-fed 5.56mm, select-fire weapon based on the Heckler & Koch HK416.  It is used by the United States Marine Corps and is intended to enhance an automatic rifleman's maneuverability.  The U.S. Marine Corps is planning to purchase 6,500 M27s to replace a portion of the M249 light machine guns currently employed by automatic riflemen within Infantry and Light Armored Reconnaissance Battalions.  Approximately 8,000–10,000 M249s will remain in service with the Marine Corps to be used at the discretion of company commanders.  The United States Army does not plan to purchase the IAR. The Heckler & Koch HK41 is a semi-automatic version of the Heckler & Koch G3 battle rifle; it was produced by Heckler & Koch for the civilian market in the 1960s.  It is not to be confused with the similarly titled Heckler & Koch G41. The Heckler & Koch SL8 is a semi-automatic rifle manufactured by Heckler & Koch.  It is the civilian version of the Heckler & Koch G36. The Heckler & Koch HK416 is an assault rifle/carbine designed and manufactured by Heckler & Koch.  Although its design is in large part based on the AR-15 class of weapons, specifically the Colt M4 carbine family issued to the U.S. military, it uses an HK-proprietary short-stroke gas piston system originally derived from the ArmaLite AR-18 (the same system was also used in Heckler & Koch's earlier G36 family of rifles).  The HK416 gained fame as the weapon that United States Navy SEALs from DEVGRU Red Squadron used to kill Osama Bin Laden in 2011. The G41 is a German 5.56×45mm NATO assault rifle introduced in 1981 and produced in limited quantities by Heckler & Koch.  It was designed to replace the 5.56mm Heckler & Koch HK33 in service providing a more modern weapon compatible with contemporary NATO standards.  It is chambered for the 5.56×45mm NATO cartridge and can use both SS109 and M193 ammunition.  Assembly of the G41 has been discontinued by Heckler & Koch, however, production rights to the rifle were acquired by the Italian arms manufacturer Luigi Franchi. The Heckler and Koch HK43 is a semi-automatic rifle based upon the Heckler & Koch HK33 rifle and is the predecessor of the Heckler & Koch HK93 semi-automatic rifle. The Heckler & Koch SL6 is a roller-delayed blowback operated sporting carbine made by Heckler & Koch.  It is manufactured in 5.56×45mm caliber.  The design was originally based on the HK 630 sporting rifle and is essentially a shorter-barreled version of that rifle.  It was marketed throughout the world as a hunting/utility rifle. The M4 carbine is a shorter and lighter variant of the M16A2 assault rifle.  The M4 is a 5.56×45mm NATO, air-cooled, direct impingement gas-operated, magazine-fed carbine.  It has a 14.5 in barrel and a telescoping stock. The SA80 is a British family of 5.56×45mm NATO small arms, all of which are selective fire, gas-operated assault rifles.  The L85 rifle variant of the SA80 family has been the standard issue service rifle of the British Armed Forces since 1987, replacing the L1A1 variant of the FN FAL.  The first prototypes were created in 1976, with production ending in 1994.  The A1 variant was significantly upgraded in the early 2000s by Heckler & Koch as the SA80A2 and remains in service as of 2017.  In mid-2016 a prototype A3 variant was showcased which further improved on the weapon and is reportedly being considered to extend the out of service date beyond 2025.
Answer: M4 carbine