Which of the following structures is not meso-$2,3$-butanediol?

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 $(Y)$.

Which types of isomers of $C_4H_8$ are possible? Give their names and structures.

The $S$-enantiomer of ibuprofen is responsible for its pain-relieving properties. Which one of the following structures shown below is $(S)$-ibuprofen?

What is the maximum number of open chain structures possible for $C_4H_8$?

Keto-enol tautomerism is found in