This is about a supply curve.

I don't understand why the rightmost point is the highest-cost barrels of oil.

My answer is the lowest-cost barrels of oil.

The reason is that as producer produce more oil, the cost to supply will decrease.

The rightmost points on the supply curve for oil represent:

My choices are

the easiest barrels of oil to extract.

the highest-cost barrels of oil.

the lowest-cost barrels of oil.

the most profitable barrels of oil.