Typically, we don't write any diacritics on "AL" because we know it is a definite article. Some books would like to add a special mark to note that this is a "hamzat-wassl" (hamza of connecting), which is an Alif that acts somewhat like Schwa (the sound of which can be eliminated or sucked in by vowels preceding it). Anyway, typing wise, adding any diacritics on the alif of "AL" is not required at all. If your answer was marked wrong because of this probably you should report it.
In case that's a maddah on top of آل, then that's never an article. In case that's a hamzat wasl (like a tiny ص), then you'll use it after vowels. If the word before ends on a vowel, you'll "smooth" things by kind of not pronouncing the alef. Otherwise you'd need a hamza (ء), but you can't just add some letters randomly.