For the statement "a is congruent to b modulo n"

Which of the following definition is correct.

a) a - b is divisible by n.

b) a - b is a multiple of n.

or whether both are acceptable.

Whether "a is congruent to a mod 0" is true?

a - a = 0 is divisible by 0 is wrong. But 0 is a multiple of 0 is correct. So whether to consider

the definition (a) or definition (b).