Among the last frontiers to be mapped was/were the

Polar regions.

Explanation:

The last frontier to be mapped was the polar frontiers and thee includes the arctic and the antarctic and they are formed by the isolated and the separate environment in the north and the south poles. And is one of the most vulnerable ecosystems of the planet and remains a sensitive indicator of the global climatic changes since the presence of the sea ice covers.

Answer (d) may increase or decrease porosity through dissolution or deposition;

The answer is d., a mountain.