Explanation:

The correct answer is option 3: 2, 3 and 4 only.

- Photosynthesis (Option 1) doesn`t add carbon dioxide to the carbon cycle. In fact, it does the exact opposite. During photosynthesis, plants absorb carbon dioxide from the atmosphere and convert it into glucose, thus removing it from the carbon cycle.

- Respiration (Option 2) adds carbon dioxide to the carbon cycle. In this process, both animals and plants convert glucose and other nutrients into energy, releasing carbon dioxide as a by-product which is added back to the atmosphere.

- Decay of organic matter (Option 3) also contributes to the carbon cycle. When plants and animals die, their bodies, wastes and residual material are broken down by bacteria and fungi, a process known as decay or decomposition. Throughout this process, carbon dioxide is produced and released back into the atmosphere.

- Volcanic action (Option 4) releases carbon dioxide stored deep within earth back into the atmosphere, thus contributing to the carbon cycle.

Thus, respiration, decay of organic matter and volcanic action (2, 3 and 4) add carbon dioxide into the carbon cycle.