It's basically a wave that moves back and forth at regular speed through matter. because of that it transfers energy through a medium (matter). however the movement of the wave is limited because of this but it can still move over long distances. so because of this these types of waves don't move far from their original point or Equilibrium position. in short think of throwing a pebble into water and seeing the little ripples from it, that's a mechanical wave. 
        
             
        
        
        
Answer:
<em>D. Molecules are made of atoms of the same element or different elements.
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Explanation:
Molecules are made up of atoms. The atoms forming the molecule can be same or different. The size and function of the molecules vary from each other. The properties and function of atoms change as and when it forms a molecule. 
The atoms combine with the help of a bond and it can be a covalent bond or ionic bond or hydrogen bond to form a molecule. The bond between the atoms gives shape to the molecule.