Concept:The Nigerian National Petroleum Corporation (NNPC) is the state-owned corporation that manages and supervises Nigeria's oil and gas industry.
Explanation:NNPC is responsible for both upstream and downstream activities in the petroleum sector.
Upstream operations involve exploration and production of crude oil.
Downstream operations involve refining, distribution, and marketing of petroleum products.
More importantly, NNPC regulates and supervises the entire oil industry on behalf of the Nigerian Government.
This regulatory role makes it the main body overseeing the development of the oil sector.
It does not primarily fix product prices as its main function, nor does it only develop oil-producing areas or just ensure regular supply.
Therefore, the main function that best matches is overseeing the development of the oil sector.
Answer:A. oversee the development of the oil sector