User enumeration is:

a. When a malicious actor can use brute-force techniques to either guess or confirm valid users in a system
b. when a malicious actor can use a Trojan virus to steal user credentials
c. when a malicious actor can use SQL injection to steal user credentials or search/verify a specific user in the system of interest
d. when a malicious actor can use a Man-in-the-Middle attack to intercept valid users data in a system