The fundamental unit among the options given is the "Second". A fundamental unit is a unit of measurement that is independent and cannot be derived from any other units. The second is a fundamental unit of time and is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom. In contrast, the other units listed are derived units, which means they are combinations of fundamental units. For example, the Newton is a unit of force, which is derived from the fundamental units of mass, length, and time.