Which literary device enhances understanding by comparing two unlike things?

The correct choice is a metaphor, which enhances understanding by drawing a direct comparison between two unlike things, suggesting that one thing is another to illustrate a point or idea more vividly. This comparison helps readers grasp complex concepts or emotions by relating them to something more familiar or tangible. For instance, saying "Time is a thief" suggests that time stealthily takes away moments from our lives, just as a thief might take possessions.

While both similes and metaphors serve to create comparisons, a simile uses "like" or "as" to make the connection explicit (e.g., "as brave as a lion"), whereas a metaphor does so implicitly, allowing for a deeper, more imaginative interpretation. Alliteration relates to the repetition of consonant sounds, and onomatopoeia pertains to words that phonetically imitate a sound, neither of which inherently involve the comparison of unlike things. Thus, a metaphor is the most fitting literary device in this context for enhancing understanding through a creative and impactful comparison.