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.

aquifers are bounded above and below by impermeable rock. the driving force of groundwater movement in confined aquifers is pressure, rather than gravity.