This is the main human activity that contributes to global
a-massive dumping of pesticides into the worlds oceans
b-high demand and depletion of the worlds oil supply
c-burning fossil fuels
d-increase of levels of chlorine poured into the shorelines

I believe the answer would be C, burning fossil fuels. Oil and gas is made up of the remains of microscopic plankton. Over millions of years the remains become the carbon-rich coal, oil and gas we can use as fuel. When fossil fuels are burned they release carbon dioxide into the atmosphere, thus contributes to global warming. 

If i’m not mistaken your answer should be the freemen’s bureau started them. i really hope i could ! good luck