Final Answer:
Twin feather bustles are one of the hallmarks of modern fancy dance regalia, along with a beaded bodice, leggings, bells just below the knees, moccasins, and other feathered or beaded accouterments.
Thus the correct option is 10
Step-by-step explanation:
In the context of describing modern fancy dance regalia, the term that appropriately completes the sentence is "accouterments." Accouterments refer to additional items of dress or equipment, and in this case, it encompasses the various embellishments like feathers or beads that complement the overall attire.
The other options, such as fortuitous, brigand, magnanimous, ubiquitous, capitulate, presage, prolixity, sinecure, accordance, viand, habiliment, extol, resplendent, bereave, and benign, do not fit the context of describing elements of fancy dance regalia.
Thus the correct option is 10