If I had the possibility to mark their answers correct and award back points, I would. But I don't have that option. And I don't think the software even caters for the possiblity of a question where two answers are marked correct (unless you require people to identify all correct answers).

However, technically speaking they are not really correct. The message you get when running out of space in an index rebuild is different from the message I gave in the answer options (which, if I recall correctly, I copied from SSMS after forcing SQL Server to run out of space while inserting rows). But given that there is only one answer in the "out of space" area and no "none of the above" option, I do believe that those 47% should have gotten their points.