What structure in ferns contains a cluster of spore-producing sporangia?

The correct answer is associated with the structure known as sori, which are found on the undersides of fern leaves, or fronds. Sori are clusters that contain sporangia, which are the structures responsible for producing spores via meiosis. These spores are essential for the reproductive cycle of ferns, allowing for the development of new plants. The presence of sori on fronds is a defining characteristic of mature ferns, making them easily identifiable during the reproductive phase.

Other choices refer to different structures in the life cycle or anatomy of ferns. Strobili, for example, are cone-like structures found in some vascular plants that also function in reproduction but are distinct from ferns. Pinnae are the individual leaflets that make up a fern frond, while prothallus refers to the gametophyte stage in the fern life cycle, which develops from the spores produced by the sporangia. Thus, while all the terms are related to ferns, sori are specifically responsible for housing sporangia and producing spores.