Have to choose between A & C. But, not sure how to prove which one is better or wrong is tough.
This is a pretty tough resolve the paradox question from what usually appears.
Let's use numbers to understand what's happening in the passage:
In the last decade,
Suppose, it used to cost $2 to produce one unit of energy from Solar power plants
We are given that price per barrel has to be minimum $35 for solar energy to be economically viable. What does economically viable means? It means that the cost of production of energy from solar power plant is not greater than the cost from oil run power plants.
So, at threshold level, cost per unit of energy from solar power plant = cost per unit of energy from oil power plant
Now, we are given that this threshold level is at oil price of $35 per barrel.
So, a barrel of oil should produce 17.5 units of energy (so that the cost is $2 per unit).
Now, coming to today,
Solar power sources have become very efficient. Therefore, let's suppose the cost of solar energy today is $1 per unit.
However, we are given that the threshold price level of barrel of oil is same i.e. $35.
This means that the same barrel oil must produce 35 units of energy for the costs of both solar energy and oil energy to remain same.
This means that the efficiency of oil power plants must have increased in same proportion as that of solar power plants.Therefore, Option C is correct.
For completion sake, let's also look at option A. It says that the cost of oil has fallen - First of all, we are not concerned about cost of oil; we are concerned about price of oil to the power plants. Even if we say that the cost here refers to the price paid by the power plants, it is still irrelevant because when we are talking of a threshold price level of oil, we are referring to a hypothetical level below which solar energy would not be viable. We are not talking about actual price level; just as we are not talking about whether solar energy is economically viable in reality or not. We are just talking of conditions that are needed to make it economically viable.
I think this is pretty tough.
Hope I made some sense.
Let me know if some further clarification is required.