The Clue Biscuit : Cookie :: Petrol : ___
The Answer GAS
The relationship in the clue shows that a biscuit is a type of cookie, just as petrol is a type of gas. In many places, petrol is commonly referred to as gas, especially in the United States. Both pairs represent a broader category and a specific example within that category. Therefore, "gas" is the correct answer that completes the analogy.