A simple, text-based MVC pattern should be implemented, which increments a counter upon user input (via the terminal). A counter is incremented whenever a user presses "return". The value that is displayed is the counter multiplied by a factor (value = counter * factor). Multiple views should be supported. The code - except the code for the model - is given as follows: 

public class Quiz4 { 
 
    static interface ModelListener {
        public void action(ModelEvent e);
    }
    
    static class ModelEvent {
        private final int counter;
        ModelEvent(int counter){
            this.counter = counter;
        }
        public int getCounter() {
            return counter;
        }
    }
    static class View {
       View(Model model,int factor) {
          ModelListener l = e -> System.out.println("Counter x factor: " + e.getCounter()*factor);
	  model.addListener(l);
       }
    }
	
    static class Control {
       public void requestInput(Model model) {
         while (true) {
          System.out.print("Press return: ");
	  try {
             // read until newline/return is pressed         
             while (System.in.read()!='\n');
	  } catch (IOException e) { e.printStackTrace(); }
	  model.setCounter(model.getCounter()+1);
        }
      }
    }
    public static void main(String[] args) {
       // One model, one controller, two views
       Model model = new Model();
       View view = new View(model,1);
       View view1 = new View(model,2);
       Control control = new Control();
       control.requestInput(model);
    }
}

Which of the following implementations of the model are correct (i.e., bug-free and supporting the required behavior)?

a)
   class Model {
        int counter = 0;
        List<ModelListener> ml = new ArrayList<>();
        
        public int getCounter() { return counter; }
        public void setCounter(int counter) {
            this.counter = counter;
            for (ModelListener l : ml) {
                l.action(new ModelEvent(this.counter));
            }
        }
        public void addListener(ModelListener l) {
            ml.add(l);            
        }        
    } 
   
b)
 class Model {
        int counter = 0;
        List<ModelListener> ml = new ArrayList<>();
        
        public int getCounter() { return counter; }
        public void setCounter(int counter) {
            this.counter = counter;
        }
        public void addListener(ModelListener l) {
            ml.add(l);        
        }        
    }
c)
    class Model {
        int counter = 0;
        ModelListener l = null;
		
	public int getCounter() { return counter; }
	public void setCounter(int counter) {
	   this.counter = counter;
	   if (l != null) {
	      l.action(new ModelEvent(this.counter));
	   }
	}
	public void addListener(ModelListener l) {
	   this.l = l;			
	}		
    }
d) 
    class Model {
        int counter = 0;
        List<ModelListener> ml = new ArrayList<>();
		
	public int getCounter() { return counter; }
	public void setCounter(int counter) {
	    this.counter = counter;
	    for (ModelListener l : ml) {
	        l.action(new ModelEvent(this.counter));
	    }
        }
	public void addListener(ModelListener l) {
	   ml = null;			
	}		
   }