Final answer:
Ferns are limited to moist habitats due to their swimming sperm which requires a moist environment to reach the female gamete for fertilization. option 3 is the correct answer.
Step-by-step explanation:
Pterophytes, such as ferns, are primarily limited to moist habitats because of their swimming sperm. Ferns, along with other seedless vascular plants like club mosses and horsetails, require water for fertilization to occur.
The male gametophyte releases flagellated sperm, which must swim in a moist environment to reach and fertilize the female gamete or egg. Despite having vascular tissue that allows them to transport water and nutrients effectively, the swimming sperm is a significant factor that necessitates a moist habitat for successful reproduction.
Other options such as small size, vascular tissue, sporangium structure, and seed dispersion are not the primary reasons for the moisture requirement of pterophytes.
Therefore, amongst the provided options in your question, the mentioned correct option in final answer is option 3) swimming sperm.