Answer
$1.8\times10^{3} kilograms.$
Work Step by Step
Volume $=Length \times Width \times\,Height=25m\times15m\times4m=1500m^{3}$
So, mass of air in kilograms $=1500m^{3}\times\,1000\,Litre/m^{3}\times1.2\,grams/Litre=1800\times1000\,grams=1.8\times10^{3} Kilograms.$