2 is CORRECT. Repeated application of the absorption theorem x + xy = x in the expression gives the desired result:
AB + ABC + ABCD + ABCDE + ABCDEF
--------
AB + ABC = AB (x = AB, y = C)
---------
AB + ABCD = AB (x = AB, y = CD)
---------
AB + ABCDE = AB (x = AB, y = CDE)
-----------
AB + ABCDEF = AB (x = AB, y = CDEF)