Which of the following adds/add carbon dioxide to the carbon cycle on the planet Earth? 1. Volcanic action 2. Respiration 3. Photosynthesis 4. Decay of organic matter Select the correct answer using the code given below.

Volcanic action, respiration, and the decay of organic matter are processes that release carbon dioxide into the atmosphere, making option (c) correct. Option (d) is the most tempting wrong choice, but it is incorrect because photosynthesis is a carbon sink that removes $CO_2$ from the atmosphere to produce carbohydrates. The core concept being tested is the distinction between sources and sinks within the Earth's carbon cycle.