Step 1 - Understanding the relation of potential energy, kinectic energy and the conservation of energy
The potential energy is the energy related to a body that can start moving under the influence of a force anytime soon.
The kinf of potential energy depends on the force that could act on the body. When a body is lifted, because gravity can pull it back, we say that this body has gravitational potential energy
Kinetic energy, on the other hand, is related to actual movement. It depends on the mass as well as on the velocity of the body. When we lift an object and drop it next, all the potential energy will be converted to kinetic energy, i.e., the velocity of the object we dropped will be also proportional to the height. It cannot gain more energy.
Step 2 - Interpreting the problem
Now that we know how potential and kinetic energy work, we can start working on the problem. Since potential energy is directly proportional to height, the highest potential energy would be attained at F.
The lowest potential energy, on the other hand, woud be attained at D, i.e., the smaller height.
The highest kinetic energy would be attained when all the potential energy is converted to kinetic energy, i.e., when the bicyclist, starting at A, loses its potential energy reaching D
The speed is directly proportional to the kinetic energy: the higher the speed, the higher the kinetic energy. Since the kinetic energy, as we saw, is the greatest at D, the velocity will also be the greatest at this point.