In the 1970s, the island of Nauru had one of the highest per capita incomes in the Oceania region due to extensive _______ mining

A. copper

B. phosphate

C.gold

D. nickel

Answer :

C. phosphate 
At the time being Nauru had one of the highest GDP per capita. However the reserves of phosphates were exhausted, which caused economic downfall. 
Government of Nauru tried to diversify it's budget be encouraging offshore banking.

Currently economy of Nauru is based on banking, fishing and 
phosphate mining (deeper layers of phosphates were found)

The correct answer is B. Phosphate

The Island of Nauru got very rich based on its phosphate deposits. (The story of Nauru is fascinating.