x y 
3 5
6 2 1/2
9 1 2/3
12 1 1/4
15 1 

a) 
b) 
c) 
d) 

We try substituting x=3 and y=5 to see which ones give true results:

a) 
    
     
   
   
   

That's false so we've ruled out b 

c) 
   
   
   

So c is true, so we haven't ruled out c yet 
     
d) 
    
   

That's false so we've ruled out d

So the correct choice is either a or c:

------------------

Let's try the easiest values:

We try substituting x=15 and y=1 to see which ones give true results:

a) 
    
     
   

That's false so we've ruled out a. So the answer is c.

Edwin