The amount of space an object takes up is called its
Explanation for correct option:
(A) The volume of an object is the amount of space it takes up.
Explanation for incorrect options:
(B) The mass of an object is the amount of matter it contains.
(C) Density is a measure of how heavy an object is in comparison to its size.
(D) The length of an object is a measure of how long or short it is.
Hence, option (A) is correct.