The small, heart-shaped gametophyte called a prothallus is part of the life cycle of ferns. So, the correct option is (b).
A fern (Polypodiopsida or Polypodiophyta) is a type of vascular plant that reproduces through spores and lacks both seeds and flowers. Vascular plants have xylem and phloem. Their vascularity, or the presence of specialised tissues that conduct water and nutrients, sets them apart from mosses and other bryophytes.
A fern's prothallus is a flat, green, autotrophic, cordate (heart-shaped), dorsiventral, free-living, autonomous, nonvascular gametophyte that ranges in size from 1/8 to 113 inches (5-l0 mm). Typically, the gametophyte stage of the life cycle is known as the protothallium or prothallus. The spore that germinates becomes the prothallium.
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