When a radioactive isotope releases an alpha particle, the atomic number of the atom is decreased by four. 
So, the correct answer is B. 
Hope this helps,
Davinia.
 
        
             
        
        
        
Answer:
Potassium selenide
Explanation:
Potassium selenide (K2Se)
 
        
                    
             
        
        
        
You are actually giving 5 points. Ps I just got five points for telling you that
        
             
        
        
        
Water or also known as H2O