Answer:
d. soil that is always frozen
Step-by-step explanation:
Permafrost is the layer of ground that remains frozen the complete year. Soil, rocks and sediments collectively together forms the permafrost. Such soil may remain frozen for a period of two or more than two years. A large part of the Earth's surface has been covered by the layer of permafrost. These are found mainly near the high mountains and especially near North and South poles.