I know it's been a long time since you asked this question, and I apologize for not being active here for a while. But if you are still interested, Here is my answer:

The for loop before this statement finds that the Element I is divisible by all the elements in array A, a[j]. Thus at any given point if any element a[j] cannot divide I, it should shut off and move on to the next element I + 1.

How do we know if all the elements in array A actually were able to divide I? - If you are still in the loop, and your j is n-1, meaning, you've reached the last element in array A (As the n is the length of A), all the elements at position (J==n-1) satisfied the division (otherwise, the loop would've exited - See j = n in else condition - that is exit logic).

Now if all a[j] were satisfying, we need to check if the element I can divide all elements in B, b[k]. The same logic used in checking if all elements in B are divisible by I - meaning if (K==m-1) where m is length of B - All elements at position m-1 were divisible by I.

At this point we found our element - So answer = answer + 1.