If Zephyr is standing on Earth and holding a helium-filled balloon on a string, what will happen to the balloon when Zephyr lets go of the string?

Respond very briefly.
The balloon will rise upwards into the sky. The reason for this is that the helium gas inside the balloon is less dense than the surrounding air. Due to buoyancy, which is a force that pushes an object in a fluid (in this case, air) upwards, the balloon will float upwards when Zephyr releases it. Gravity pulls everything towards the center of the Earth, but since the balloon is buoyant relative to the denser air, it rises instead of falling like most other objects.