Which action involves placing items into specific categories?

The action that involves placing items into specific categories is categorising. This process entails sorting or organizing objects, ideas, or pieces of information based on shared characteristics or criteria. This is essential for understanding relationships among different entities and aids in comprehending complex information by simplifying it into manageable sections.

In contrast, assessing involves evaluating or measuring the quality, importance, or value of something, which does not inherently involve sorting into categories. Applying entails using knowledge or skills in a practical context, while clarifying focuses on making something clear or easier to understand, rather than organizing it into groups. Therefore, categorising distinctly addresses the task of classification, making it the most appropriate answer.