In the high arctic, the ground at sea level is frozen as

Permafrost

Explanation:

Permafrost is perennially frozen ground that does not thaw on an annual basis. It can only exist in geographic regions of the Earth that are normally very cold, such in the High Arctic or in mountains.

In fact, permafrost can only exist where mean annual temperatures are at 32 degrees Fahrenheit (or 0 degrees Celsius).

Of course, the coldest areas of the earth are either the high mountains (like the Andes or the Alps) or the high/low arctic/antarctic latitudes (such as Alaska or Siberia). Because this question is asking about frozen ground at the sea level in the High Arctic, they are most likely asking about permafrost.

a and b because with the other three, they simplify to something else.