Final answer:
The infinitive in the sentence is 'to eat' and it is being used as an adverb to modify the adjective 'happy', explaining why the hogs are happy.
Step-by-step explanation:
The infinitive in the sentence 'Hogs are happy to eat.' is 'to eat'. This infinitive is functioning as an adverb. It is modifying the adjective 'happy' by explaining why the hogs are happy. In other words, it tells us the reason for the hogs' happiness is the activity of eating. Therefore, the correct answer is 'to eat, used as an adverb'.
It's important to understand that an infinitive phrase can act as a noun, adjective, or adverb. In the given sentence, because it's providing more information about the adjective, it is adverbial, not nominal (noun) or adjectival.