The following structures are:

An aqueous solution containing compounds $A$ and $B$ shows optical activity. $A$ and $B$ are stereoisomers. Which of the following possibilities cannot be correct?

What is the process of conversion of a pure optical enantiomer into an optically inactive mixture of $(+)$ and $(-)$ enantiomers called?

The following compounds are best described as:
$(R)-PhCH(OH)CH_3$ and $(S)-PhCH(OH)CH_3$

How many meso isomers are possible for $2, 3, 4-\text{pentanetriol}$?

Which of the following combinations amongst the four Fischer projections represents the enantiomers?