The correct answer is option 2: at the maximum height.

To understand why the potential energy is maximum at the maximum height, it is important to understand the concept of potential energy. Potential energy is the stored energy an object possesses due to its position or state.

When the body is thrown vertically upwards, it gains potential energy due to the force applied to lift it against gravity. As it continues to ascend, the potential energy increases because its height from the ground increases. At the maximum height, the body momentarily stops moving upward and starts falling back towards the ground.

During the return journey, the body loses potential energy as it descends and gets closer to the ground. The potential energy gradually decreases until it becomes minimum when the body reaches the ground again.

Therefore, the potential energy of the body is maximum at the highest point of its trajectory, which is the maximum height. So, option 2 is the correct answer.