Allusions are defined as
a. two concepts that have nothing in common being put together for description.
b. types of figurative language that are used to state informative facts and ideas.
c. verses that are written without end rhymes and without any kind of strict meter.
d. references to things, places, people, and events outside of the reading selection.

The correct answer would be option D.

Allusions are defined as references to things, places, people, and events outside of the reading selection.

An allusion is a literary device which uses a short or indirect reference to an event, place, or person. It is pre-assumed by the writer that the readers must be acquitted with the reference used in the work. Allusions can be used to enhance the text by adding additional meaning to it. At times, it can also be used to make ironic comments by comparing the dissimilar things.