The correct answer is option 1: a gas whose density is lower than air. When a balloon is filled with gas, it rises in the air because it is buoyant. Buoyancy is the force that allows an object to float or rise in a fluid, like air.
The density of an object determines its buoyancy. Density is a measure of how much mass is packed into a certain volume. If the density of an object is lower than the density of the fluid it is in, it will experience an upward buoyant force and float or rise. On the other hand, if the density of an object is higher than the density of the fluid it is in, it will sink.
In the case of a balloon, the gas inside should have a lower density than the surrounding air for the balloon to rise. If the gas has a density higher than that of air, the balloon will sink. Option 1 correctly states that the gas in the balloon should have a lower density than air for it to go up in the air.
Alert - correct answer should be option 2: a gas whose density is higher than air. The gas filled in the balloon should be denser than the surrounding air for the balloon to sink.