What are four advantages the plant gains by the miniaturization of the gametophyte?

Answer 1

their tiny gametophytes can develop from spores retained within the sporangia of the parental sporophyte. this arrangement protects the delicate female (egg-containing) gametophytes from environmental stresses

So

D. megaspores for PF students.

Answer 2

Increased cross fertilization and great dispersal of the next generation are the advantages of multicellular pollen and seeds.

Spores are the way ferns and moss spread longer distances. Spores are single-celled and vulnerable .so dispersal is limited. Being tied to water, as moss & ferns are with flagellated gametes, makes self-fertilization more common so genetic recombination is at a low rate.

Gymnosperm and angiosperm plants -keep the spore- until it matures as the entire male gametophyte or pollen. This groups them as Spermatophytes with multicellular pollen rather than single celled spores. Essentially the gametophyte phase is reduced to the brief period of reproduction. Male cones or male flower parts release the miniature gametophyte as the grain of pollen. Female cones or flower parts grow the female gametophyte in an ovule. In cones the ovule is sheltered by the cone's scale but is not enclosed. In flowers the ovule is inside an ovary.

Spermatophytes have no flagellated gametes to restrict these plants to water for reproduction. With the mature, miniature, male gametophyte arriving as pollen the sperm nuclei are delivered to the egg nuclei to form a zygote. This allows -much- more recombination of genetic material with the mobility of pollen. Then the seed is the juvenile sporophyte that is also mobile & capable of dispersal.

These plants are much less vulnerable to reproductive losses while ensuring cross-fertilization and dispersing to new locations.