The most common elements present in organic compounds are carbon, hydrogen, oxygen, and nitrogen. 
 
        
                    
             
        
        
        
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<u>HOP</u><u>E</u><u> IT</u><u> </u><u>HELPS</u><u> </u><u>YOU</u>
 
        
        
        
Answer:
 X is negative heterotropic modulator
Explanation:
In allosteric regulations, modulators are molecules that causes a change in the conformation of an enzyme, hence, resulting a change in enzyme activity. It can lead to a decrease or an increase of the enzyme.  When a molecule decreases the enzyme activity it called a negative modulator, when it increases or activates the enzyme activity, it is called a positive modulator.
A positive or negative modulator can either be homotrophic (substrate acts as modulator) or heterotropic (another ligand acts as modulator).
In the example above, X is negative heterotropic modulator, because the modulator is different from the substrate and it also decreases the enzyme activity.