smack=defeat

"That, though no fault of his own, George Cuvier, the father of extinction theory, has mostly slipped into obscurity, his name typically surfacing only in paleontology journals, is an outcome that—given his field and his claim that his work would endure—smacks of _____."

Lets filter all the adjectives here. I find "not his fault", precedence, obscurity, not popular, would endure, in that order. Now let's see the answer choice.
unexpectedness
arrogance
magnanimity
irony
insignificance

Quick guess would result to irony, because someone who has precedence over a study should be popular, yet because of the subject is too obscure for other discipline, he is not.

I selected irony but how can we eliminate unexpectedness ?

The best word to fill in the blank in the sentence is "irony."

George Cuvier, the father of extinction theory, expected his work to endure and be well remembered. However, despite his significant contributions, he has mostly slipped into obscurity outside specialized paleontology journals. This outcome, which contrasts sharply with his own expectations, fits the concept of "irony"-a situation where the actual result is opposite or strikingly different from what was anticipated.

So, the sentence reads well as: "...is an outcome that-given his field and his claim that his work would endure-smacks of irony."

None of the other options (unexpectedness, arrogance, magnanimity, insignificance) capture this contrast between expectation and reality as effectively.