Final answer:
The presence of an embryo retained within the plant's tissue would identify an organism as a land plant, as this is a characteristic feature not found in algal groups, including green algae such as Chlorophytes and Charophytes.
Step-by-step explanation:
The presence of an embryo retained within the plant's tissue would definitively identify the organism as a land plant. This is because, unlike algae, land plants retain their embryos inside the parent plant's tissues. Additionally, while green algae such as Chlorophytes and Charophytes have many structural similarities with land plants, they do not protect or retain their embryos in this way. Unlike land plants, green algae do not develop through an embryonic stage that remains attached to the parent plant; therefore, the presence of an embryo is a unique feature that identifies the organism as a land plant. Other features listed, such as spindle fibers, sporopollenin, and lignin, could be present in algae and therefore do not exclusively indicate a land plant.