Which is the BEST definition of Half-Life?

Question

a
The half-life of a substance or element is the amount of time it takes for half of the initial quantity of that substance or element to decay or transform into another substance.
b
The half-life of a substance or element is the amount of energy it takes for all of the initial quantity of that substance or element to decay or transform into another substance.
c
The half-life of a substance or element is the amount of matter it takes for all of the quantity of that substance or element to decay or transform into another substance.

Answer 1

The BEST definition of Half-Life is:

a
The half-life of a substance or element is the amount of time it takes for half of the initial quantity of that substance or element to decay or transform into another substance.