Final answer:
Licensure is the mandatory credential for certain health professionals to practice in their fields. It is issued by government agencies and it validates a professional's qualifications and legal eligibility to practice.
Step-by-step explanation:
The credential that is mandatory for certain health professionals to practice in their field is Licensure (D). Licensure is typically issued by government agencies and it certifies that a worker has completed required education, training, and has often passed a relevant examination. Without proper licensure, professionals such as doctors, teachers, nurses, engineers, accountants, and lawyers are not legally permitted to practice. The concept of credentialism underscores the importance of credentials like licenses, certificates, or degrees in demonstrating a professional's qualifications and skills within the labor market.