Final answer:
The subject of the Pentheus fresco in the House of the Vettii is mythology, depicting a mythological narrative possibly involving the Greek king Pentheus and the god Dionysus.
Step-by-step explanation:
The subject of the Pentheus fresco in the House of the Vettii in Pompeii, Italy, reflects the theme of mythology. This ancient artwork belongs to the broader category of Roman wall paintings, which thanks to the preservation afforded by the volcanic ash from the eruption of Mount Vesuvius, allow us to have a glimpse into ancient Roman life and beliefs. The House of the Vettii is known for its elaborate frescoes, which include themes from Roman mythology and are examples of the detailed and vibrant art from this historical period. The specific fresco of Pentheus would depict a mythological narrative, likely involving Pentheus, a king in Greek mythology who was associated with the tragic stories surrounding Dionysus and his followers, the Bacchae.