Regarding system high availability, what is the reference used to determine reliability?

Regarding system high availability, what is the reference used to determine reliability?
A . the amount of data available to users and how quickly they can access it, measured in IOPS
B . the number of hard drives available in a system, with more drives indicating more reliability
C . the number of data network components available between the data and the user, measured in packet per second (PPS)
D . the number of nines; for example, two nines (99), three nines (99.9), four nines (99.99), and five nines (99.999)

Answer: D