Concept:Form an equation using the relationship between the two numbers and their given sum.Explanation:Let the larger number be x.According to the question, the smaller number is one-fourth of the larger number.So, the smaller number =4x​.The sum of the two numbers is 45.Thus, x+4x​=45.Combine the terms: 44x+x​=45.So, 45x​=45.Multiply both sides by 4: 5x=180.Therefore, x=5180​=36.Now, the smaller number =4x​=436​=9.Answer:The smaller number is 9, which corresponds to option D.