a. Li - Element (Li represents Lithium, which is a chemical element with atomic number 3.)
b. CO - Compound (CO represents Carbon Monoxide, which is a chemical compound composed of carbon and oxygen atoms.)
c. Cf - Element (Cf represents Californium, which is a chemical element with atomic number 98.)
d. CF₄ - Compound (CF₄ represents Carbon Tetrafluoride, which is a chemical compound composed of one carbon atom and four fluorine atoms.)