Final answer:
The figurative language in the sentence is a simile, which compares the heat of the leather seats to that of lava using the word 'as'.
Step-by-step explanation:
The figurative language used in the sentence: "The car had been sitting in the sun all afternoon, so the leather seats were hot as lava," is a simile. A simile is a figure of speech that compares two different things by using the words "as" or "like." In this case, the heat of the leather seats is being compared to lava, emphasizing the extreme temperature the seats have reached due to being in the sun.