Which of these is a compound?

О

co2,

O2,

Co

co2 is a compound.

Explanation:

Oxygen (O) is an element having atomic number 8 and mass number 16.

Carbondioxide is a compound formed by burning of carbon and organic compound and it is a colorless and odorless gas.

O2 is the molecular form of Oxygen.

Cobalt (Co) is an element having atomic number 27 and mass number 59.