If a and b are integers such that a > b > 1, which of the following cannot be a multiple of either a or b?
(A) a – 1
(B) b + 1
(C) b – 1
(D) a + b
(E) ab

The answer is C.

I am confused by C can’t be the multiple.
For example, if a: 12, b: 7, it is 12>7>1, and b-1 is 6. then can’t b(6) be a multiple of 12?

I think I’m thinking something wrong.
What am I confused here and what concept am I missing?

I think you’re confusing the definition of what is a multiple . In your case 12 is a multiple of 6 and not 6 is a multiple of 12(6 is a divisor/factor of 12).

To elaborate further :
The lowest multiple of a number will be the number itself. Hence, the lowest multiple of 12 will be 12 and lowest multiple of 7 will be 7.
The first 3 multiple of 12 will be 12,24,36.

Ahh thank you for the explanation.
I am confused by words like multiples, factors, divisible, etc.
Could you please recommend me any better ideas than just memorizing concepts?