Final answer:
The phase of risk assessment described is 'c. Exposure assessment', which determines the populations at risk and the nature of their exposure to toxicants.
Step-by-step explanation:
The component of risk assessment that identifies the population exposed to a toxicant and describes their composition is c. Exposure assessment. This part of the risk assessment process involves determining which individuals or communities are at risk, the nature and size of the exposed population, their specific characteristics, and the duration and intensity of their exposure to the toxicant. Understanding the extent of exposure, alongside other components such as hazard identification, dose-response assessment, and risk characterization, is essential in determining the potential health risks posed by a chemical or toxicant.