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.