Which of these organelles produces h2o2 as a by-product?

The parts constituting a single cell are called its organelles. Examples are ribosomes, mitochondria, endoplasmic reticulum, Golgi bodies and many more. You might confuse the answer to this question because there is an organelle named Peroxisome. It is called such because it synthesizes Hydrogen Peroxide or H₂O₂. It breaks the compound into its individual components, water and oxygen. So, it uses up H₂O₂, not produce it. The organelle that actually produces H₂O₂ as a by-product is the mitochondria through the process of respiration.