Let's see that in the options, carbon monoxide (CO) and lead (II) sulfide (PbS) are toxic compounds to the human body, so we have to discard these options.
Now, we have oxygen molecule (O2) and sodium chloride (NaCl), and these compounds are essential to the human body. But, NaCl is an ionic compound because there are two ions (cation and anion) combined chemically. On the other hand, we have that oxygen molecule which is two oxygens atoms that are bonded, it represents a covalent bond because they are sharing their electrons, so the answer is oxygen molecule.
