In this scenario, you are measuring the growth of bean plants (specifically, the number of leaves) over time (every three days for six weeks). Since you have continuous data (number of leaves) collected at regular intervals (time), a line graph would be the best choice to display your data.
A line graph allows you to clearly show trends and changes over time, making it easy to see how the number of leaves varies with each observation.
So the answer is: Line graph.