The most likely answer to this question would be D. chemical bonds. Atoms are held together in a molecule by the chemical bond that it has formed. When atoms are together, they usually form a chemical bond to stay together. These bonds differ from atom to atom as to what kind are they, their nature, and all of the factors present in an element. When such factors are taken into consideration, chemical bonds happen from time to time. And during such bonds, they are also separated when there is another factor that causes them to be separated.
