The correct answer is option 1: troposphere. The atmosphere is divided into several layers, with the troposphere being the lowest and closest to the Earth`s surface. It extends from the Earth`s surface up to an average height of about 8-15 kilometers. This layer is where weather phenomena occur, such as clouds, rainfall, and thunderstorms. It contains most of the Earth`s air mass and is composed mainly of nitrogen, oxygen, and other trace gases. As you move higher in the atmosphere, you encounter other layers like the stratosphere, mesosphere, thermosphere, and exosphere. The troposphere is essential for sustaining life on Earth as it allows for the exchange of oxygen, carbon dioxide, and other gases that are vital for living organisms.