Answer:
D. The relationship between volume and moles
Explanation:
Amedo Avogadro found the relationship between the volume of a gas and the number of molecules contained in the volume. 
The law states that "equal volume of all gases at the same temperature and pressure contains the same number of molecules or moles".
Mathematically: 
                          V ∝ n (P,T constant)
 
        
             
        
        
        
Nitrogen fixation is the process that makes atmospheric nitrogen available to plants by mutualistic and free-living bacteria. The process is undertaken by the rhizobium bacteria that live in root roots of plants such as legumes. The mutualistic relationship is that the plant supplies the bacteria with a habitat in which to live, water, and nutrients, and the bacteria supply nitrogen for making plant proteins. 
        
             
        
        
        
Sodium metal is quite reactive; sodium ions (as in NaCl) are quite unreactive. Cl^- ions are not reactive; they are stingily attracted to positive ions such as Na^+with which they form ionic bonds.
        
             
        
        
        
Answer:
The correct answer is - scientific law.
Explanation:
A scientific law is an explanation to a well observed phenomenon of the world which is found true every time tested and universal proven fact, but the reason behind the phenomenon and its existence.
To explain the scientific law there will be various explanation that are called scientific theory, however these theories can not be proved.
Thus, the correct answer is - scientific law.