What could be put to XXX to prevent Thread collisions? : Questions « Thread « SCJP

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class Board {
   Image[] pics = new Image[64];
   int active = 0;
   public boolean addPic( Image mg,int pos ){
     synchronized(XXX)
     { if( pics[pos] == null ){
        active++ ; pics[pos] = mg;
         return true;
       }
       else 
         return false;
     }
   }


   1. synchronized( this )
   2. synchronized( pics )
   3. synchronized( mg )
   4. synchronized( active )