The best visual element to accompany the excerpt about the gigantic cave discovered in Canada would be a video showing the first views of the cave. This option would provide the audience with a direct and engaging experience, allowing them to see the cave's immense size and unique features firsthand. Additionally, a video can capture the awe and wonder of the discovery, making the presentation more dynamic and memorable.
While the other options offer value—such as providing context or comparisons—visualizing the cave itself in a video format would create a stronger impact during the speech.