Transition words and phrases play a crucial role in enhancing the clarity of a comparison-contrast essay primarily by showing how ideas are related. They help guide the reader through the essay by clearly indicating connections between different points, making it easier to follow the writer's argument or analysis.
While transition words can also influence the emphasis placed on certain points, their primary function is to clarify relationships between ideas—whether comparing similarities or highlighting differences. Therefore, the best answer is:
They show how ideas are related.