Which of the following is a reducing sugar?

Markovnikov's rule is applicable to:

Name the class of enzyme that usually catalyze the following reaction : $\mathrm{S}-\mathrm{G}+\mathrm{S}^{\#} \rightarrow \mathrm{~S}+\mathrm{S}^{\#}-\mathrm{G}$ Where, $G \rightarrow$ a group other than hydrogen $S \rightarrow$ a substrate $\mathrm{S}^{\#} \rightarrow$ another substrate

Identify the suitable reagent for the following conversion 

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : undergoes $\mathrm{S}_{\mathrm{N}} 2$ reaction faster than Reason (R) : Iodine is a better leaving group because of its large size. In the light of the above statements, choose the correct answer from the options given below :

Total number of possible isomers (both structural as well as stereoisomers) of cyclic ethers of molecular formula $\mathrm{C}_4 \mathrm{H}_8 \mathrm{O}$ is :