Which background process is responsible for issuing alerts when the threshold is crossed?

You have set Tablespace Full Metrics Threshold values for the USERS tablespace as follows:
Warning (%): 90
Critical (%): 95
Which background process is responsible for issuing alerts when the threshold is crossed?

A.
Manageability Monitor process (MMON)

B.
Process monitor (PMON)

C.
Memory manager process (MMAN)

D.
System monitor (SMON)