No force contributes to density, the density is a physical quantity that is defined as being 

 the raport between the mass of the object and its volume. However if you want to measure the density of an object you might want to determine its gravity force (weight) 

 from which knowing the gravitational acceleratin you can find its mass 
 
 where 

 is given in Newtons and 

 is given in  
 
 
        
        
        
<span>The sun, lightning, the ionosphere, fluorescent light bulb, stars, fire, tv, and solar winds. I don't know your other options but these are my examples.</span>
        
             
        
        
        
Explanation:
We know that the number of complete waves formed in 1 sec time is frequency and the distance between two consecutive crests or troughs is wavelength. And we have the formula that
Velocity = wavelength * frequency 
or, frequency = velocity / wavelength 
Here we can see frequency is directly proportional to velocity and indirectly proportional to wavelength. 
So as the wavelength increases frequency decreases and as the wavelength decreases frequency increases. 
Hope you understood 
 
        
                    
             
        
        
        
The is true because I said so
        
                    
             
        
        
        
I am not completely sure, but I believe that it depends on the total mass of the Protons and Neutrons