He cycle repeats when the carbon stored in the atmosphere as carbon dioxide gas is taken in.

carbon dioxide is taken in by plants during photosynthesis. the cycle repeats when the carbon stored in the atmosphere as carbon dioxide gas is taken in. the animal releases the carbon dioxide back into the atmosphere during respiration. the animal eats a plant and uses its carbohydrates for energy

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.