Answer-The gas used for filling weather balloons is-

Helium is used for filling weather balloons because 1. Helium gas is lighter than air and it has lesser density as compared to the air. So, we observe that the balloon starts rising upwards and this is because of its less density than air i.e. about one-eighth times lesser than air. 2. It is inert and non-flammable and therefore does not react with air or surroundings or gets explode.