[list:1337] [clues] ●There is only 1 person who ate the cake ●The criminal can't fail to tell a lie ●Anyone except the criminal might tell a lie or tell the truth
If "A" is the criminal, "A" come to tell a truth. It contradicts the clues. "B" and "D" can be excluded for the same reason. As a result, remaining "C" is the criminal.