Answer:
C: Sustenance is used as an adjective, but it is a noun.
Explanation:
The word "sustenance" in the sentence is a noun that is being used as an adjective for another noun "food". Adjectives have the function of modifying a noun in a sentence, but in the English language it is also possible to use a noun with the function of an adjective , in the position of an adjective before a noun.