What is the designation for the 60mm mortar used by U.S. infantry?

The designation for the 60mm mortar used by U.S. infantry is M224A1. This mortar is lightweight and portable, enabling infantry units to provide indirect fire support while remaining mobile. The M224A1 features a smoothbore tube and is designed for rapid deployment and firing. It can overcome obstacles and accurately deliver explosive ordnance to support ground forces engaged in combat.

The other options presented refer to different weapons systems: the M203 is an under-barrel grenade launcher; the M240 is a medium machine gun; and the MK19 is an automatic grenade launcher. Each of these has distinct roles and capabilities within the infantry squad, but none of them serves as a 60mm mortar, making M224A1 the correct answer in the context of this question.