The most abundant enzyme in the living world is Rubisco, which is the correct answer. Rubisco, short for Ribulose-1,5-bisphosphate carboxylase/oxygenase, is an enzyme involved in the photosynthesis process. It is found in plants, algae, and some bacteria. Rubisco is responsible for the fixation of carbon dioxide from the atmosphere during photosynthesis, converting it into organic compounds. This enzyme is crucial for the production of glucose, which is the primary source of energy for living organisms. Rubisco is so abundant because it is found in almost all photosynthetic organisms, and photosynthesis is a fundamental process for sustaining life on Earth. Its abundance and importance in the global carbon cycle make Rubisco the most abundant enzyme in the living world.