What term describes the stage in ferns after the spore germinates?

After a spore of a fern germinates, it typically develops into a structure known as the prothallus. This small, heart-shaped gametophyte is the first stage in the life cycle of ferns that arises from the germinating spore. The prothallus is a crucial part of the fern's reproductive process, as it produces both male and female gametes necessary for fertilization. Once fertilization occurs, it will eventually give rise to the sporophyte generation, which is the dominant phase in ferns.

The terms associated with the other choices relate to different aspects of fern biology. While the sporophyte is the diploid phase that develops from the zygote of the fertilized egg, the frond refers specifically to the leafy part of the mature sporophyte. A rhizome is a type of underground stem that serves as a storage organ and a means of vegetative reproduction, but it does not describe the stage after the spore germinates. These distinctions highlight the unique stages in the fern life cycle and clarify why the prothallus is the correct term for the stage that follows spore germination.