The correct answer is "unicorn, fuel" because both words contain the long "u" sound. In "unicorn," the "u" is pronounced like "yoo," and in "fuel," the "u" is pronounced like "yoo" as well. The other words, "mum" and "sum," do not have the long "u" sound. "Mum" has a short "u" sound, pronounced like "uh," and "sum" has a short "u" sound, pronounced like "uh" as well.