Final answer:
To rewrite a ≡ b (mod n), we use the equation a = bn + r, which expresses the congruence relation correctly, indicating that a and b leave the same remainder when divided by n.
Step-by-step explanation:
To rewrite the expression a ≡ b (mod n) into a workable equation, understand that this congruence relation means that a and b have the same remainder when divided by n. Therefore, the correct form of the equation that represents this relationship is a = bn + r, where b is a quotient, n is the modulus, and r is the remainder.
The answer is therefore option b) a = bn + r.