Answer:
Metaphor
Explanation:
You haven't provided the options you were given and I can't find them online, but I can answer your question anyway.
The figure of speech used in the given quote from Emerson's Nature is a metaphor. Just like the simile, the metaphor is a figure of speech used to compare two unrelated things that share some qualities. The difference between these two figures is that simile represents a more direct comparison. It uses the words as and like, while the metaphor doesn't. It states that something is something else.
What Emerson wants to say is that in good health, the air is like a delicious and healthy drink.