Blunderboy said:-Snip-
This one is tricky because technically the correct answer is spell, though in game you want to answer sword as to avoid an actual fight. Here's the logic from UESP:cloudywolf13 said:-Snip-
If you answer that the atronach will kill you with a spell, he will demonstrate that it is indeed the truth by attacking you. If, instead, you tell the atronach a lie - that he will slay you with a sword, he proves its falsehood by letting you go.