In the enolization of the given molecule,the $H$-atom involved is

Which of the following pairs represents stereoisomerism?

Which one of the statements about $HOH_2CCH(OH)CHO$ is not correct?

The correct stability order of the given tautomers is:

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 $(Y)$.

The correct order of the heats of combustion of the above compounds is: