Dear Student,
i) According to kinetic theory of gases, no attractive or repulsive forces act between gas molecules. Therefore, the gas molecules can move freely and thus occupy all the spaces available to them .
ii) Since there is no force of attraction or repulsion between molecules of gas, therefore, they move freely and can attain the shape of the vessel in which they are contained. Therefore, they donot have fixed shape.
iii) Gases exert pressure due to the molecular collision taking between between molecule of gas and the walls of the container. All of these collisions are elastic and no loss of energy occur.