What is an example of onomatopoeia, personification, and simile in the poem "The Road Not Taken"?
This is a tricky question, because many of the elements that you ask about are not actually in the poem.
There is no onomatopoeia in this poem. Onomatopoeia is when the sound of a word imitates the actual sound to which it refers. An example of onomatopoeia might be "bang" or "achoo!" The word "bang" sounds like the sound of a bang, while the word "achoo" sounds like a sneeze.
You may have incorrectly identified sigh as onomatopoeia in this poem. The word "sigh" is generally not considered to be onomatopoeia because the sound of the word does not resemble the actual sound of a sigh.
Personification occurs in the line "because it was grassy and wanted wear." Personification is basically ascribing human traits to an inanimate object, idea, or animal. In this case the speaker is describing the road as having a desire, which is a trait normally attributed to people.
There are no similes in this poem. A simile compares two very different things by using the words "as" or "like". An example of a simile might be, "My cousin is as cheerful as a morning bird." Two dissimilar things - my cousin and the morning bird - are compared using the word "as".
You might spot the phrase "as just as fair" in this poem and wonder if this could qualify as a simile. Because you are comparing two very similar things, in this case two roads, this does not constitute a simile.