Which of the following statements about permafrost is true?

(a) it is rare in periglacial environments.
(b) permafrost that thaws and refreezes produces a pentagonal or hexagonal ground surface pattern called columnar joints.
(c) it can extend as deep as 1,500 km below ground surface.
(d) it can melt to a depth of a few meters in summer, then refreeze in winter.

D. It can melt to a depth of a few meters in summer, then refreeze in winter.

Explanation:

Permafrost is ground which remains frozen for two or more years. Permafrost consists of soil, rock and sand fused together by ice. Mostly found in areas in high altitude, the Arctic, Antarctica, it is common in periglacial environments where temperatures are below freezing .

The topmost layer of permafrost can melt in summer and refreeze in winter.  This thawing and refreezing produces a ground pattern called columnar joints similar to the one formed when molten lava cools.