What is the possible number of stereoisomers for the compound $CH_3-CH=CH-CH(OH)-CH_3$?

What is the relationship between the molecules in the following pairs?

Which of the following pairs of compounds is a pair of enantiomers?

Which of the following types of isomerism is shown by pentanone?

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 $(Y)$.

Among the given compounds,the correct order of enol content is: