The correct answers are The brave elves sharpened their knives., Autumn leaves fell as the wild wolves howled., and The royal chefs prepared a grand feast for the king.

For most nouns ending in -f or -fe (like elf, knife, leaf, wolf, thief, loaf), we change the f to a v and add -es (elves, knives, leaves, wolves, thieves, loaves).

However, there are a few exceptions, like chef, which simply takes an -s to become chefs.