Final answer:
The velocity of the roller coaster at the top of the loop is at its highest point because potential energy is converted into kinetic energy.
Step-by-step explanation:
In the bar graph, the highest bar represents the roller coaster's potential energy at the top of the loop. As the coaster descends, some of the potential energy is converted into kinetic energy, resulting in an increase in velocity. Therefore, the velocity of the roller coaster at the top of the loop is at its highest point due to the conversion of potential energy to kinetic energy.