B is correct. However this is a really fine detail that I'm sure some native speakers would get wrong. - It is generally true that the lower the stock market falls, the higher the price of gold rises. OR - It is generally true that if the stock market were to fall, the price of gold would rise - It is generally true that how low the stock market falls will determine how high the price of gold will rise There are many ways to say it, but just make sure that the tenses match up in a statement like this.

I would say "b" is correct.