A(n) is a large parcel of the atmosphere, sometimes subcontinental in size, that may move over earth's surface as a distinct, relatively homogenous entity.

The answer is:  "air mass" .

        "An    air mass    is a large parcel of the atmosphere, sometimes subcontinental in size, that may move over earth's surface as a distinct, relatively homogenous entity."

Ithink the answer to that would be b):

Physical systems is the answer