Which term describes the main body of the balloon that holds lighter than air gas?

The term that describes the main body of the balloon which holds the lighter-than-air gas is the envelope. The envelope is a vital component of a balloon, designed to contain the gas—typically helium or hot air—that allows the balloon to become buoyant and rise into the atmosphere. It acts as a large bladder that expands as the gas fills it, creating the lift necessary for the balloon to ascend.

Understanding the other terms helps to clarify why the envelope is the correct answer. The gondola is the structure that carries passengers or payload and is suspended beneath the envelope; it does not hold the gas but rather provides a platform for the balloon's operation. The crown refers to the top section of the envelope, often where the gas escapes or where vents are located, rather than the entirety of the balloon's gas-holding component. Panels typically refer to the individual fabric segments that construct the envelope, but themselves do not serve as a descriptor for the whole gas-holding body. Hence, the envelope is the precise term for the entire structure that contains the lighter-than-air gas.