Leonora invests $4,500 and earns 2% in simple interest each year assuming she makes no additional deposits or withdrawals,how much will her investment be worth in 6 years

There are no variables being divided. There are no worries about square roots or anything like that. So we have no restrictions to worry about. The domain is the set of all real numbers. Therefore, the answer is D. We can replace x with any number we want to get some output for f(x).