I would say carbon monoxide
Firstly because the image shows two different atoms bonded so it cannot be nitrogen or krypton
Secondly sulphur dioxide has 3 atoms bonded (two oxygens and a sulphur atom) so it can’t be that
Finally it can’t be hydrogen chloride because chloride is significantly larger than hydrogen
Thus it must be carbon monoxide as carbon and oxygen are bonded (CO) and are both relatively similar in size
