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.

Explanation:

By controlling the synthesis of enzymes, DNA controls the functioning of the cell. Of the four different bases taken three at a time, there are 43, or 64, possible combinations. The meaning of each of these combinations, or codons, is known. Most of them represent one of the 20 particular amino acids found in protein.