Which structures allow the flow of carbon dioxide into a plant?

The correct answer is: stomata

Stomata are microscopic openings-pores located on the surface of a leaf (sometimes on stems) necessary for gas exchange. The opening and closing of pores are regulated processes thanks to guard cells that surround stomata. Stomata allow carbon dioxide take in, for photosynthesis but they also help in reducing the water loss by closing when conditions are hot or dry.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.

condensation. good luck!