Where are the spore-containing sori of a fern found

on the underside of fertile fronds/leaves

Explanation:

In ferns, the sorus (plural: sori) refers to a cluster of sporangia which are usually found on the undersides of fertile fronds.

Sporangia are spore producing and housing structures found in ferns and some other plants.

Spores are propagules or reproductive cells that are capable of giving rise to new individuals either singly or in conjunction with another spore.

Leaves in ferns are generally referred to as fronds and depending on the species of fern, there exists fertile fronds, sterile fronds or a combination of the two. Fern species with both sterile and fertile fronds are known as dimorphic ferns.

Ferns reproduce both sexually and asexually depending on the species. Sexual reproduction is by haploid spores which are usually produced and housed in the sori.

These sori are arranged on the undersides of fertile fronds of ferns.