The correct answer is D) enigma.

Breakdown of the Blank

The sentence states that "none have been able to decode" the ancient tablet. The word that fills the blank must describe something that is impossible to figure out.
- Enigma: This means a person or thing that is mysterious, puzzling, or difficult to understand. This word perfectly fits the description of an ancient tablet that no one can decode.

Let's look at why the other options are incorrect:
- A) gem: This means a precious stone or something outstandingly good.
- B) stanza: This refers to a group of lines forming the basic recurring metrical unit in a poem.
- C) quagmire: This means a soft boggy area of land that gives way underfoot, or a difficult, awkward, or hazardous situation. While the tablet could be a "difficult situation" for a researcher, "enigma" is a more precise and accurate term for a puzzle.
- E) incentive: This means a thing that motivates or encourages someone to do something.