The carbon cycle is the movement of carbon on Earth by the processes of --- and ---- .

Carbon exists in the atmosphere as the compound carbon dioxide. It first enters the ecological food web when photosynthetic organisms, such as plants and certain algae, absorb carbon dioxide through tiny pores in their leaves. The plants then "fix" or capture the carbon dioxide and are able to convert it into simple sugars like glucose through the biochemical process known as photosynthesis. Plants store and use this sugar to grow and to reproduce. Thus, by their very nature as makers of their own food, plants remove carbon dioxide from the atmosphere. When plants are eaten by animals, their carbon is passed on to those animals. Since animals cannot make their own food, they must get their carbon either directly by eating plants or indirectly by eating animals that have eaten plants.

Respiration is the process in which oxygen is used to break down organic compounds into carbon dioxide  (CO2) and water (H2O). For an animal then, respiration is both taking in oxygen (and releasing carbon dioxide) and oxidizing its food (or burning it with oxygen) in order to release the energy the food contains. In both cases, carbon is returned to the atmosphere as carbon dioxide.

So, the correct answer is 'Respiration and photosynthesis'.