The correct answer is - d. They are concerned that too much oil production will cause the price of oil to go down, resulting in lower profits.
The countries that are members of OPEC are trying to control the supply of oil on the global market with the simple purpose to manage to get as bigger profit as possible.
It is a simple economic rule, that if there is too much of a certain product on the market, its prices drop, and if there's less from a certain product on the market, its prices remain high. That is the tactic that the OPEC members are using, and so far is working very well for them, which can be seen by their development in the past few decades.
