Final answer:
The Fourth Labor of Hercules involved capturing the Erymanthian Boar and he encountered the Centaurs during this quest.
Step-by-step explanation:
In the Fourth Labor, Hercules took on the Erymanthian Boar. While searching for it, he had an encounter with the Centaurs. Therefore, the correct answer to fill in the blanks would be: a) Boar; Centaurs. Hercules is a key figure in Greek mythology known for his strength and for the Twelve Labors he was compelled to perform. The labor involving the Erymanthian Boar was a task in which Hercules was tasked with capturing a fearsome boar that was terrorizing the region around Mount Erymanthus. During his quest, an encounter with the wine-drinking Centaurs led to a conflict that further demonstrated Hercules's might and capabilities.