Which one is the most stable cation in the following?

Which of the following compounds does not exhibit keto-enol tautomerism?

Which of the following is not a resonance structure of the others?

What are the following compounds called?

Which one of the following is most stable?

The correct statements with respect to the above pair of reactions are that:
$(I)$ The reactions are stereospecific.
$(II)$ $(X)$ is erythro and $(Y)$ is threo isomer.
$(III)$ $(X)$ is threo and $(Y)$ is erythro isomer.
$(IV)$ Each of $(P)$ and $(Q)$ gives a mixture of $(X)$ and $(Y)$.