a) volume of water to total volume of voids
b) volume of air voids to volume of total voids
c) volume of air voids to volume of water
d) volume of voids to total volume of the soil mass

Answer: a
Explanation: In a soil sample, some space is occupied by water and the rest by air. The degree of saturation is defined as the ratio of the volume of water to the total volume of voids.