The question asks which factor is vital in the development of the seed habit. A) Haplontic life cycle: This life cycle is characterized by a dominant haploid stage and is not associated with seed plants. Seed plants have a dominant diploid sporophyte. B) Heterospory: This is the production of two different types of spores: microspores (male) and megaspores (female). Heterospory is considered a crucial evolutionary step towards the seed habit because it leads to the retention of the megaspore within the sporangium, which eventually develops into the ovule and then the seed. C) Free-living gametophyte: In seed plants, the gametophytes are highly reduced and dependent on the sporophyte, not free-living. Free-living gametophytes are characteristic of non-seed plants like ferns. D) Dependant sporophyte: In seed plants, the sporophyte is the dominant, independent plant body. The gametophyte is dependent on the sporophyte. Therefore, heterospory is vital for the development of the seed habit. The final answer is $\boxed{\text{B}}$ That's 2 down. 3 left today — send the next one.