Got it right by elimination. There is no way a NULL value would be converted to an actual date (as far as I know, at least), so that eliminates answer B. Date types can't return an empty string (that would have to be char, varchar, etc.), so that eliminates answer A.
However, I never memorized the date that is equivalent to 0 and I didn't know if datetime, datetime2 and date had different conversions from integers, so if there had been an answer such as NULL, 1900-01-01 00:00:000, 1753-01-01 00:00:000, 1900-01-01, I might had gotten it wrong. Guess I learned that much from this question, then.
Thanks, Sasidhar Pulivarthi! :-DEdit: changed "don't know if datetime (...) have different conversions from integers" to "didn't know", which made me realized I actually learned something new here.