Concept:Translate the word problem into a linear equation and solve for the unknown number.Explanation:Let the unknown number be x.Subtracting x from 12 gives 12−x.Multiplying this result by 3 gives 3(12−x).The problem states that this equals 21, so we write 3(12−x)=21.Divide both sides of the equation by 3 to get 12−x=7.Now subtract 12 from both sides: −x=7−12=−5.Multiply both sides by −1 to obtain x=5.Check the answer: 12−5=7, and 7×3=21, which matches the given condition.Therefore, the unknown number is 5.Answer:The number is 5, which is Option C.