Which of the following molecules do not exist?

Given below are two statements:
Statement $I$: $F_2O < H_2O < Cl_2O$ is the correct trend in terms of bond angle.
Statement $II$: $SiF_4, SnF_4$ and $PbF_4$ are ionic in nature.

Considering ethene molecule,which of the following statements is/are correct?
$I$. $C-C$ bond is formed by overlapping of $sp^2-sp^2$ hybrid orbitals.
$II$. $C-H$ bond is formed by overlapping of $sp^2-s$ orbitals.
$III$. $p\pi-p\pi$ bond is present.
$IV$. $H-C-H$ bond angle is greater than $H-C-C$ bond angle.
$V$. Nodal plane of $\pi$ bond and molecular plane are same.

Identify the correct statements from the following.
$(I)$ $SnCl_2$ is ionic,but $SnCl_4$ is covalent in nature.
$(II)$ All linear diatomic molecules have zero dipole moment.
$(III)$ Both $NO$ and $O_2$ are paramagnetic.

Which of the following cations is the most stable?

In which species,$X-O$ bond order is $1.5$ and contains $p\pi - d\pi$ bond$(s)$?