Question

How are professional credentials usually maintained? A. By joining the professional association B. By completing a continuing education cycle C. By receiving a degree D. By passing an annual examination

Answer 1

I think that "A" is correct, joining the professional associations is very helpful and effective for the "maintenance" of every skills and every knowledge and education.

Answer 2

How are professional credentials usually maintained?
 A. By joining the professional association

but the answer B may also be appropriate
B. By completing a continuing education cycle