An allusion is an indirect reference. An illusion is a false appearance. An allusion to the opening scene. An optical illusion. Diction as a set is in word choice.
Reference versus trick
If the sentence is about pointing at another text, person, or event, use allusion. If it is about something that looks real and is not, use illusion. The verb pair is in allude versus elude.
Imply versus infer is in imply versus infer. Grammar overall is in ACT grammar rules.
The bottom line
Indirect reference: allusion. False appearance: illusion.
Start practicing
Start with a free diagnostic, then drill your weak spots with 15-question quizzes and track how you're doing across Reading, English, and Math. Compare plans whenever you're ready to go further.
This article offers general ACT prep guidance. The ACT can change from year to year, including its format, scoring, policies, test dates, and fees, so always confirm the latest details on the official ACT website at act.org before you make decisions. ACT® is a registered trademark of ACT, Inc. thirty-six is not affiliated with, endorsed by, or sponsored by ACT.