Option (c) Troposphere is correct. According to the textbook, the troposphere is described as 'the most important layer for all biological activity.' This layer contains dust particles and water vapour, and all changes in climate and weather take place in this layer. Since human beings are biological organisms, the troposphere is the atmospheric layer most important for us. The context also states that 'geographers are concerned with the first two layers of the atmosphere' (troposphere and stratosphere), but specifically emphasizes the troposphere's importance for biological activity.