The correct answer is A. 40%.
The degree of saturation of a soil is defined as the ratio of the volume of water in the soil to the total volume of voids in the soil. The air content of a soil is defined as the ratio of the volume of air in the soil to the total volume of voids in the soil.
If the degree of saturation of a partially saturated soil is 60%, then the air content of the soil is 40%. This is because the total volume of voids in the soil is 100%, and the degree of saturation is 60%, so the volume of water in the soil is 60%, and the volume of air in the soil is 40%.
Option B is incorrect because the degree of saturation cannot be greater than 100%. Option C is incorrect because the air content cannot be greater than 100%. Option D is incorrect because the air content cannot be equal to 100%.