Question

fix the one word that is used incorrectly.
in one famous optical allusion, two circles of the same size are surrounded by images that trick the viewer into seeing one circle as larger than the other.

Explanation:

"Allusion" means a passing or indirect reference, while "illusion" means a false perception or deceptive appearance. In the context of an optical phenomenon that tricks the viewer, "illusion" is the correct word.

Answer:

Change "allusion" to "illusion".