Question

what is permafrost?
snow or ice that builds up above ground during the winter.
a temporary mixture of snow and ice on the ground.
a subsurface layer of the ground that remains frozen for at least two years in a row.
the top most layer of a glacier that is exposed to the air/atmosphere

Explanation:

Permafrost is defined as a subsurface layer of the ground that remains frozen for at least two consecutive years. The first option describes seasonal snow or ice accumulation, not permafrost. The second option refers to a temporary mixture, which is not permafrost (permafrost is long - term). The fourth option is about the top layer of a glacier, not the subsurface frozen ground characteristic of permafrost.

Answer:

A subsurface layer of the ground that remains frozen for at least two years in a row.