The answer to 15 is 4 because cellular respiration in the process that produces ATP. Protein synthesis, photosynthesis, and digestion don't directly produce ATP. Carbon dioxide is a by-product of cellular respiration.
Looking at your answer to number 16, I think you might have confused cellular respiration with "regular respiration" or breathing. We breathe in oxygen and breathe out carbon dioxide because the mitochondria in our cells require oxygen to produce ATP, and release carbon dioxide as a byproduct.
Therefore, number 16 isn't cellular respiration. It's not happening within a cell, it's not producing ATP, and mitochondria aren't involved. Instead, it takes place in the lungs (because the alveolus is part of the lungs). You can see that carbon dioxide is being removed from the blood and oxygen is being added. This is the process of gas exchange.
Check out this video for more info on cellular respiration: https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/overview-of-cellular-respiration-steps/v/overview-of-cellular-respiration
And this one for more about gas exchange: https://www.khanacademy.org/science/health-and-medicine/respiratory-system/gas-exchange-jv/v/oxygen-movement-from-alveoli-to-capillaries
Hope that helps! Feel free to ask if you need clarification or anything :)
