Which of the choices is an example of an element?
O CO
O Co
O C2H2O2
O CO2

Co(cobalt) is the answer

Explanation:

Cobalt is a chemical element with the symbol Co and atomic number 27. Like nickel, cobalt is found in the Earth's crust only in chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal.

the atomic mass is the bottom number on the element.

-steel jelly

the atomic number will stay the same no matter how many electrons it has, so it doesn’t decrease nor increase. it’ll only change with a different number of protons