That will be A melting a substance that's because the rest are examples of chemical changes and also its the same thing but in a different form.
        
                    
             
        
        
        
A.Positive as work=force×distance (assuming that distance is always a constant) the work will always be positive as the force is always positive(this is because force=mass×acceleration where the acceleration is always positive unless the mass is being slowed down)