Which of the following defines the atmosphere?

The atmosphere is defined as a layer of gases surrounding a planetary body, which is precisely what makes option C the correct choice. This layer is made up of various gases, including nitrogen, oxygen, carbon dioxide, and trace amounts of others, which are crucial to maintaining life on Earth by providing the necessary elements for respiration, weather patterns, and even protection from harmful solar radiation.

The atmosphere plays an essential role in regulating temperature through the greenhouse effect, allowing Earth to sustain life. Furthermore, it is essential for weather and climate systems, which are vital for the ecosystems and human activities on the planet. This definition directly relates to how the atmosphere functions and its significance in environmental science.

In contrast, the other options define different aspects of Earth's systems; a layer of soil relates to the lithosphere, while a solid outer part of the Earth corresponds to the crust. The last option, a measure of biodiversity, pertains to the variety of life forms in ecosystems, which is a separate concept not directly linked to the definition of the atmosphere.