The blank requires a word that connects two actions happening at the same time: "breathing air out" and "eating."
While (Correct): This is a conjunction/preposition used to indicate that two actions are occurring simultaneously. In this context, as you eat, you also breathe out, which allows your sense of smell to contribute to the perception of flavor.
In: Usually indicates a position or a period of time (e.g., "in the morning"), but it doesn't fit the action of eating here.
From: Indicates a source or a starting point, which is logically incorrect for this sentence.
With: Usually indicates accompaniment or the tool used. While we eat with a fork, we don't breathe air out with the act of eating in this grammatical structure.