Which of the following is not an element? a. carbon b. hydrogen c. oxygen d. salt

The right answer is D. salt.

In chemistry, a salt is an ionic compound of cations and anions forming a neutral product and without a net charge. These ions can be as well inorganic (Cl- chloride) as organic (CH3-COO- acetate) and monoatomic (F- fluoride) as well as polyatomic (SO4 (2-) sulfate).

idk

explanation:

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.