Which type of mining is likely the least harmful to the environment?
a. surface mining
b. placer mining
c. subsurface mining
d. none of the above

First of all, all types of mining will bring negative impact for environment, 

but The type of mining that is likely the least harmful to the environment is : C. Subsurface mining

Surface mining : Will cause contamination of soil and groundwater which may lead to loss in biodiversity

Placer mining : Will cause destruction within Fishes and Fishes habitats