I have written the values of the variables before and after the statement gets executes and all side effects have taken place.
int a[5] = {5, 1, 15, 20, 25};
int i, j, m;
// before : i is indeterminate, a[1] = 1
i = ++a[1];
// after: i = 2, a[1] = 2
// before: j indeterminate, a[1] = 2
j = a[1]++;
// after: j = 2, a[1] = 3
// before: m indeterminate, i = 2
m = a[i++];
// after: m = a[2] = 15, i = 3
Therefore, the final values are
i = 3
j = 2
m = 15