Final answer:
A bar graph is the most appropriate data display method for comparing the photosynthetic activity of the plants when investigating the effect of varying carbon dioxide levels.
Step-by-step explanation:
The most appropriate data display method for comparing the photosynthetic activity of the ten plants when investigating the effect of varying carbon dioxide levels would be a bar graph.
A bar graph provides a visual representation of different variables, in this case, the photosynthetic activity of the plants, and allows for easy comparison between different carbon dioxide levels. The x-axis can represent the different carbon dioxide levels, while the y-axis can represent the photosynthetic activity.