"Vested benefits

a. usually require a certain minimum number of years of service.
b. are those that the employee is entitled to receive even if fired.
c. are not contingent upon additional service under the plan.
d. are defined by all of these."

are defined by all of these(D)

Explanation:

Vested benefit is a financial package that is given to an employee who has fulfilled the term of service that is necessary to receive a full benefit. As a way of making employees stay with a company, the employers may offer the employees benefits in which, the full amount is gradually acquired, as they spend more time with the company.

When the worker has earned the full rights to the benefits after a required number of years of service, the benefits are called vested benefits.