About one-third of the toxic metal, mercury (Hg), is released into the air from natural sources, such as rocks, soil, volcanoes, and the ocean. The remaining two-thirds come from human activities such as . a. airplane exhaust b. gold mining c. livestock farming d. automobile exhaust e. plastic recycling

Two-third of the toxic metal, mercury comes from gold mining.

Explanation:

During gold mining small pieces of gold which are within  the soil are separated with the help of mercury. Both gold and mercury are mixed and they forms amalgam. Further gold can be extracted by vaporizing the mercury, thus releasing it into air and contributing to two-third of the air pollution by human activities.

Therefore (d) gold mining is the correct answer.