18) E

Set up algebraic equations for any of the elements present. 
e.g. Cu on the left side appears ‘a’ times. Cu appears ‘x’ number of times on the right side. 


a = x
However, all of the options has a = x. (Option A has a = 2, x = 2)


Let us try N
b = 2x + 2
Try substituting the values of ‘b’ and ‘x’ from the options given. 
Option A cannot be correct. 
Option B cannot be correct.
Option C can be correct.
Option D can be correct. 
Option E can be correct


Let us make an equation for O

3b = 6x + y + 2

Try substituting the values of b, x and y from the options given. 
Option A cannot be correct. 
Option B cannot be correct.
Option C cannot be correct.
Option D cannot be correct. 
Option E can be correct


So E is the correct answer.