The best type of visual aid to explain the legislative process would be:
D. a flowchart showing the passage of the bill in both houses and then moving on for the president's approval.
A flowchart is effective in illustrating processes and sequences, making it easy to follow the steps involved in the legislative process, including how a bill is introduced, debated, amended, and ultimately passed in both the House of Representatives and the Senate before being sent to the president for approval. This option provides a clear and visual representation of the flow of the legislative process.