Student or Learner
The ancient Scandinavians venerated the mistletoe because it ...... brought good luck and fertility.
A much more natural and meaningful sentence would be: The ancient Scandinavians venerated mistletoe because they believed (that) it brought good luck and fertility.
In BrE, I don't find #2 acceptable at all. Just because a group of people thought (assumed/presumed) that something was true, that doesn't mean we can use "presumably".
I would say that none of them fit. I would reword it "... because it was thought to bring good luck and fertility". At a push, I might say "... because it was presumed to bring ...". I would not use "presumably" in that sentence at all.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
It's a poor question. The answer that fits the sentence best for me, apart from being factually wrong, is C).
As an AmE speaker, I would also say presumably works acceptably here. Not elegant, not quite le mot juste, but okay, and clear enough. Something like allegedly, or reportedly, or supposedly.