First off, this isn't really one of our questions. Someone took an example sentence from our SC guide and decided to add answer choices to it, so there's no official answer. However, C mirrors the version we used in our guide.
If "price" were the
subject of the sentence, B would be objectively wrong. We can see that easily by removing the adjective "oil":
The prices have gone down.
Prices have gone down.
The price has gone down. Price has gone down. 
In this way, "price" isn't much different from any other noun. For instance, we can say "Dogs are running," but not "Dog is running."
However, in this sentence, "price" is part of a modifier ("in price"), and it's fairly normal to say things like "A drop in price would entice more buyers." So can we say "an increase in oil price"? I wouldn't, but people do. I can't say definitively whether the GMAT would accept this. However, given the choice between B and C, C is safer.