None of the above. It's a common misconception that the word "weeks" needs an apostrophe when being described by worth, however as the use of apostrophes is highly limited, it doesn't. Therefore, "It's disappointing that three weeks worth of assignments WERE lost." The "were" is necessary unless a specific noun of participle-imperfect relation were applied after the usage of "was lost".
I suppose if you have to pick one of the above, choose C, and explain to your teacher that it's not really correct because of the implication of an apostrophe and correct the use of was/were.
Hope that helped more than confused.
