Shannon feels miserable in her marriage but refuses to consider getting a divorce; she feels that a divorce would completely waste all of the hard work she has put into making the marriage work for 20 years. Shannon's line of reasoning best illustrates the:

sunk-cost fallacy

Explanation:

Sunk-cost fallacy: In psychology, the term "sunk-cost fallacy" is described as a phenomenon that an individual commits when he or she tends to continue a specific behavior or else endeavour as a consequence of formerly invested different resources including effort, time, or money.

Example: A individual buys a movie ticket or a cloth which is non-refundable.

In the question above, Shannon's line of reasoning best illustrates the "sunk-cost fallacy.