Final answer:
The best relative pronoun to complete the sentence is 'which'. Brine pools are underwater lakes which contain high concentrations of salt and methane gas.
Step-by-step explanation:
The best relative pronoun to complete the sentence is which.
Brine pools are underwater lakes which contain high concentrations of salt and methane gas.
Here, which is used to introduce a relative clause that provides additional information about the brine pools.
Learn more about relative pronouns