If 18:X = X:8, Then X=?

Answer: (D) 12 

Solution:-
18: X = X: 8 
Then, 18/X = X/8 
By cross multiplication, 
18 x 8 = X x X
X^2 = 144
Square root on both site, 
X = 12 

Then, the value of X is 12. 

Hence, the correct answer is option (D) 12.