Concept:The relationship is based on subtracting a fixed number from the first number to get the second number. Explanation:For 871:550, subtract 321 from 871 to get 550. Similarly, for 630:309, subtract 321 from 630 to get 309. Apply the same operation to the third pair: 397−321=76. Thus, the missing number is 76. Answer:76 (Option C)