Explanation:

The diagram presents three rows of numbers, and we need to find the missing number in the third row. Let's denote the numbers in each row as $(C_1, C_2, C_3)$. \nRow 1: $(43, 11, 6)$\nRow 2: $(50, 31, 9)$\nRow 3: $(42, 21, ?)$ \nStep 1: Analyze the relationship in Row 1.\nLet's try various arithmetic operations. Consider the sum of the first two numbers divided by the third number: $(C_1 + C_2) \div C_3 = (43 + 11) \div 6 = 54 \div 6 = 9$. \nStep 2: Analyze the relationship in Row 2 using the same pattern. $(C_1 + C_2) \div C_3 = (50 + 31) \div 9 = 81 \div 9 = 9$. \nStep 3: Identify the consistent pattern.\nBoth Row 1 and Row 2 follow the pattern: $(C_1 + C_2) \div C_3 = 9$. \nStep 4: Apply the pattern to Row 3 to find the missing number.\nFor Row 3: $(42, 21, ?)$ $(42 + 21) \div ? = 9$ $63 \div ? = 9$\nTo find '?', we calculate $? = 63 \div 9 = 7$. \nStep 5: Evaluate the options.\nOption A: $7$. This matches our calculated result.\nOption B: $8$. This is incorrect.\nOption C: $5$. This is incorrect.\nOption D: $4$. This is incorrect. \nTherefore, the number that replaces the question mark is $7$.