When is a verbal warning most appropriate?

Question $7$ options:

a$)$

when an employee has received coaching, not improved, but the problem is not severe

b$)$

when an employee has received verbal warnings, not improved, but the problem is not severe

c$)$

when an employee has committed misconduct that is serious but does not warrant termination

d$)$

when an employee is new and doesn't fully understand performance expectations