the SI UNIT for mass is what
kg
kg is correct. Here is a good site for SI units. http://physics.nist.gov/cuu/Units/units.html
The SI (International System of Units) unit for mass is indeed the kilogram (kg). The kilogram is the base unit of mass in the SI system and is widely used for measuring the mass of various objects.
To find this answer, you could have also consulted a reliable source such as a physics textbook or an authoritative online resource like the National Institute of Standards and Technology (NIST) website. The NIST website provides comprehensive information on SI units, including the kilogram. It is always a good idea to refer to trusted sources when seeking accurate information.