The correct answer is option 1, carbon dioxide. When carbon dioxide (CO2) is released into the atmosphere, it acts as a greenhouse gas. This means that it traps heat from the sun and contributes to global warming. As the temperature of the earth increases, the temperature of the oceans also rise. When water temperature rises, water molecules become energized and move further apart, causing the water to expand. This expansion of water in the ocean is one of the key factors contributing to sea level rise.

Options 2, 3, and 4 (nitrogen dioxide, carbon monoxide, and sulfur dioxide) are not responsible for the expansion of water in the ocean. Nitrogen dioxide and sulfur dioxide are also greenhouse gases, but their emissions are not as significant as carbon dioxide. Carbon monoxide is primarily produced from the incomplete burning of fossil fuels and does not directly contribute to the expansion of water in the oceans.

It is important to note that while the given answer is generally correct, it is simplified. Other factors can also contribute to the expansion of water in the ocean, such as the melting of ice sheets and glaciers, and changes in ocean currents.