The second number in the given number-pairs is obtained by performing certain mathematical operation(s) on thefirst number. The same operation(s) is/are followed in all the number-pairs, except one. Find that odd number-pair.(NOTE: The relation should be found without breaking down the numbers into its constituent digits)784 : 961

In this question following logic is followed- 

(a) 784 : 961 → 28² : 31² = 31 - 28 = 3 

(b) 1764 : 1849 → 42² : 43² = 43 - 42 = 1

(c) 1156 : 1225 → 34² : 35² = 35 - 34 = 1 

(d) 1521 : 1600 → 39² : 40² = 40 - 39 = 1 

Thus, the difference of the root of the number is same except of option (a). 

Therefore, the correct answer is (a).