Part 1;
The answer is; Carbon dioxide. Approximately 0.8% of the atmosphere is composed of carbon dioxide. It is in the gaseous phase.
Part 2;
The answer is; Combustion. Human activity of burning fossil fuels increases the carbon dioxide in the atmosphere. This has a secondary consequence of warming up the planet because carbon dioxide is a greenhouse gas.
Part 3:
Carbon is conserved in the cycles mostly in plants. Plants take up carbon dioxide in photosynthesis and make organic compounds (carbohydrates). Therefore first acts as carbon sinks. Even the fossils fuels that we burn come from plankton that is major carbon sinks too.
Part 4;
Carbon dioxide in the atmosphere is created by several processes including combustion and respiration. All living things respire out carbon dioxide. Plants take up this carbon dioxide and sequester it. When animals feed on plants they release part of this carbon back to the atmosphere through respiration