Final answer:
The figurative language in the sentence is 3) personification, which attributes human traits to non-human objects.
Step-by-step explanation:
The figurative language being used in the sentence 'His hair flopped down over his forehead in exhaustion.' is personification.
Personification is a form of figurative language where human traits are attributed to non-human objects or abstract ideas. In this case, the hair is described as 'flopping down' due to 'exhaustion', as if the hair itself is capable of feeling tired, which is a human characteristic.