Chemical bonds
a. hold atoms together to form compounds.
b. are always covalent.
c. are always ionic.
d. combine compounds to form elements.

The best answer is A.

A chemical bond  is said to be a lasting attraction occurring  between atoms that causes them to form chemical compounds.

A bond that results from an attraction of atoms due to the opposite charges (electrostatic charges) on them is called an ionic bond. When two atoms share electrons , they get bonded together in a type of bond called a covalent bond. 

Covalent and ionic bonds are the strongest types of chemical bonds that occur between atoms.

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