The correct answer regarding the literary device used in the phrase "honest air" is personification.
Personification is the giving of human characteristics to non-human/non-living things.
A metaphor is a comparison between two things not using the words "like" or "as" in the comparison. If one uses "like" or "as" then the comparison would be considered a simile.
Given that air is not a human, air simply cannot be honest. Only humans can be honest. Example of personification, so that you are able to identify the devices more easily are:
whispering trees, screaming tides, laughing grasses, waving trees.
Examples of metaphors would be:
Life is a highway (comparison of life and a highway).
She is a pig (comparison between a woman/girl and a pig).
To change the phrase "honest air" into a metaphor, one would do the following: the air is honest Abe.
"Honest air" is an example of personification, because it gives the air the quality of being honest, which is a human characteristic.