Suppose arraylist list1 is [1, 2, 5] and arraylist list2 is [2, 3, 6]. after list1.addall(list2), list1 is

The addAll(Collection c) method, simply appends c's elements to the end of the collection that the method was called from.

for example:

List<int> list1 =  new ArrayList<>(Arrays.asList(1, 2, 5));
List<int> list2 =  new ArrayList<>(Arrays.asList(2, 3, 6));
list1.addAll(list2);

for (int num : list1) {
    System.out.print(num + " ");
}

The output of the above code-snippet is: 1 2 5 2 3 6

The correct answer to go in the blank would be a) complement.