Answer:
C and E i think
Step-by-step explanation:
 
        
             
        
        
        
no. of boys : no.of girls
100 : 140
to get the simplest ratio you gotta divide both with a common factor 
Because both 100 and 140 share a common factor of 20 
the simplest ratio will be
100/20 : 140/20
5:7
5:7 is the simplest ratio
 
        
                    
             
        
        
        
Here is your solution.. hope it helps