I Think it is Carbon Monoxide Because of the Following Reasons :
From the Picture, We can Notice that the Both Atoms are Approximately of Same Size or Diameter. Carbon and Oxygen have Approximately Same Diameter.
It Cannot be Nitrogen, Because the Color of Atoms is Different. So, They must be of Different Element's Atoms.
It Cannot be Krypton, Because Krypton is a Noble Gas it does not Pair up with other Atoms.
It Cannot be Hydrogen Chloride, Because There is a Big Difference between the Size of Hydrogen and Chlorine Atom.
It Cannot be Sulphur dioxide, Because Sulphur dioxide Contains 3 Atoms. One Sulphur and Two Oxygen Atoms.