The correct answer is (a) Respiration.
Respiration is a metabolic process that releases energy by breaking down glucose. This process consumes oxygen and produces carbon dioxide as a waste product.
Photosynthesis is a process by which plants use sunlight to convert carbon dioxide and water into glucose and oxygen. This process is the primary way that carbon dioxide is removed from the atmosphere.
Decay of organic matter is a process by which bacteria and other microorganisms break down dead plants and animals. This process releases carbon dioxide into the atmosphere.
Volcanic action is a process by which volcanoes release carbon dioxide into the atmosphere.
Therefore, respiration is the only process that does not increase the amount of carbon dioxide in the carbon cycle.