An air mass must be:

(A) homogeneous and small in size
(B) uniform in the vertical dimension but may contain different temperatures and moisture contents in the horizontal direction
(C) found only in the stratosphere
(D) large, uniform in the horizontal direction and travel as a unit

Option (D)

Explanation:

In the field of meteorology, an air mass is usually defined as the large volume of air that is more or less uniformly oriented in a horizontal manner and can travel from one particular region to another under the influence of local and global wind, causing different types of weather conditions.

These air masses are comprised of a definite temperature and amount of water vapor or moisture content, and depending on these factors, they are classified as hot, cold, dry and wet air mass.

Thus, the correct answer is option (D).