Select the option that is related to the third number in the same way as the second number is related to the first number and the sixth number is related to the fifth number.

72 : 108 :: 84 : ? :: 102 : 153

126

Pattern followed here is :

1st No . :  1st No . + 1st No ÷ 2

We have ,

72 : 108

72 :  72 + 72÷ 2

72 : 72 + 36

72 : 108

&  102 : 153

102 :  102 + 102÷ 2

102 : 102 + 51

102 : 153

Similarly ,

84 : ?

84 :  84 + 84 ÷ 2

84 : 84 + 42

84 : 126

126 , is correct answer .