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

12 : 216 :: ? : 729 :: 24 : 1728 

18

12 : 216 :: ? : 729 :: 24 : 1728

The logic followed here is

(X ÷ 2 )³

12 : 216

(12 ÷ 2 )³ =  6³ = 216

and

24 : 1728

(24 ÷ 2 )³ =  12³ = 1728

therefore

? : 729

(18 ÷ 2 )³ =  9³ = 729

18 is the number which is half of 9 and 9³ = 729