Poets generally use allusion to a. disguise their political views b. make readers think about how much progress humankind has made c. deepen a poem's meaning d. display their knowledge of various fields

I think the correct answer is: C) Deepen a poem's meaning.

Allusion refers to a brief and indirect fererence to a person, a place, a thing or an idea. They are based on the assumption that most of people share a specific body of knowledge, in that way the reader undestands that reference. In that way a short allusion may lead the reader to a much deeper theme, like mithology or the bible, the poem has now a conection with something else. Sometimes the reader will understand the allusion and sometimes he won't. By making a research he will be able to understand and the poem will have a diferent of deeper mening.