Select the option that is related to the third number in the same way as the second number is related to the first number.
36 : 216 :: 81 : ?

Given: 36 : 216 :: 81 : ?

Logic: In each pair, the first number is the square and the second number is the cube of the same natural number.

(6)$$^2$$ = 36; (6)$$^3$$​ = 216

(9)$$^2$$​ = 81

So, 729 is the missing number.

Thus, the correct answer is (b).