Answer:
Option B, Less dense
Step-by-step explanation:
As we move up, the space in which gas molecules are confined increases as compared to the space near the earth's surface. Hence as we move up, the same number of air particles and molecules occupies a larger space and hence their density reduces.
Mathematically,

The higher the volume (V) will be the lower will the density (D), given mass (M) is constant.
Hence, option B is correct.