Which of the following represents an element?
со
h2co3
02
он-

Co (assuming there's a typo in not capitalizing the first letter)

Explanation:

The elements are depicted in the periodic table. Those are fundamental atoms that are not bonded to anything else but exist in their natural states without forming a bond with anything. That said, we're interested in a monoatomic species listed (the species which consist from only one single atom).

Out of all the choices, we have Co, cobalt, which is a single atom and can be found as an element with the atomic number 27.

Carbonic acid, , consists of 3 different elements: hydrogen (atomic number of 1), carbon (atomic number of 6) and oxygen (atomic number of 8). It's a compound rather than an element, as it consists of several atoms bonded together.

Oxygen, , is also a molecule. It consists of more than a single atom, even though the two atoms are identical. We have a compound, oxygen gas, which consists of two same oxygen (O) atoms (or elements).

Hydroxide anion, , also consists of two elements: oxygen and hydrogen. It also contains a charge on it, so rather than being a molecule, this is now an ion. Since it consists of more than a single atom, this is not an element as well.