What type of reproduction do ferns use

They both use a reproductive strategy called alternation of generations, where there are two different reproducing morphologies: a diploid sporophyte, and a haplod gametophyte. The sporophyte produces spores by meiosis (2N - > N), which disperse and form the gametophyte, which produces gametes by mitosis (N - >N). Two of those gametes fuse to create a diplod zygote (N + N - > 2N), which becomes the new sporophyte

There are differences between the two types of plants, especially the relative size of the sporophytes and gametophytes, but they both use this kind of reproductive strategy.