Among the given statements only I and III are correct and II is incorrect. All linear diatomic molecules does not have zero dipole moment. e.g. HF, HCl,HBr etc. Hence, statement II is incorrect. According to Fajan's rule the central metal with more oxidation state will be considered to be more covalent molecule than that have low oxidation state. In SnCl2 and SnCl4 the oxidation state of Sn is +2 and +4 respectively. Thus, SnCl4 is more covalent. Due to the presence of one unpaired electron in NO and two in O2, both are paramagnetic in nature.