I'm pretty sure the answer is the third one!
        
                    
             
        
        
        
Think about it, 16 years old? You become a adult when you turn 18. You would also have to be more responsible because 18 is the age you're actions have huge consequences. If you're 16 the worst that could happen is that you get grounded!
        
                    
             
        
        
        
I would say A.) the assertion and the reason are both correct, and the reason is valid.