Final answer:
Certification is the lowest type of professional credential for counselors, being a voluntary credential demonstrating specific skills or knowledge, as opposed to mandatory licensure.
Step-by-step explanation:
The lowest of the four types of professional credentials counselors may obtain is certification. Unlike licensure, which is often a mandatory credential that legally allows professionals to practice, certification is typically a voluntary credential that demonstrates a specific level of skill or knowledge in a professional area. Registration is a process that entails being placed on a list of practitioners who have met certain minimal qualifications, and inspection is not a type of professional credential.