The cognitive levels C1 to C6 refer to Bloom's Taxonomy, a hierarchical framework for classifying educational learning objectives. C1 represents 'Remembering,' the most basic level involving recall of facts. C2 is 'Understanding,' where learners comprehend the meaning of information. C3, 'Applying,' involves using knowledge in new situations. C4, 'Analyzing,' requires breaking down information into its component parts. C5, 'Evaluating,' involves making judgments based on criteria. Finally, C6, 'Creating,' is the highest level, requiring learners to generate new ideas or products. This taxonomy is widely used in education to design curricula and assessments.