The correct answer is puzzle.
D. cipher machine
The correct answer is (A) Puzzle; mystery; riddle. A puzzling or unfathomable person, thing, situation, or occurrence. Something that is difficult to understand fully. An obscure saying or a picture, question, or saying, etc., which contains a hidden meaning.
Enigma comes from Latin aenigma, Greek ainigma (from Greek ainisseshai [to speak in riddles]).
Examples of use:
- What he really felt remained an enigma to her.
- Why they suddenly changed their plans is still an enigma today.