You caught us red-handed if you picked answer B, "thief." "Furtive" has a shadowy history -- it may have come directly from Latin, or it may have sneaked into English through French. But however "furtive" got into English, the Latin word "fur," meaning "thief," is at the root. "Fur" may come from the Greek "phor" which also means "thief." It's not surprising that the root word of "furtive" means "thief," since a thief must behave in a furtive manner to avoid getting caught in the act!