Animals present in the food web (an interconnected network of food chains within an ecosystem) perform cellular respiration (oxidation of food to derive energy and release carbon dioxide and water) and release carbon dioxide in the environment as a waste product.
Plants utilise this carbon dioxide along with water (in the presence of sunlight) to synthesise their own food. This process that is called photosynthesis.
Thus, the correct answers are- carbon dioxide and photosynthesis.
