Answer:
kilogram is a fundamental unit because it is one of the 7 fundamental units that serve as a building block for derived units.
Step-by-step explanation:
Fundamental units are used to measure fundamental quantities. They are fundamental units because they serve as a building block for derived units.
7 fundamental quantities and their corresponding units;
Length – meter 
Mass – kilogram 
Time - seconds 
Temperature - Kelvin 
Amount of substance - Moles 
Electric Current - Ampere 
Luminous intensity - Candela
 
Thus, kilogram is a fundamental unit because it is one of the 7 fundamental units.