Correct Answer - A
The aldehyde group of an aldose reacts with carbonyl reagents such as hyroxylamine and phenylhydrazine. With hydoroxylamine, the product is the expected oxime. However, with enough phenylhydrazine, three molar equivalents of phenylhydrazine are consumed and a second phenylhydrazone group is introduced at `C2`. The product is called a phenylosazone.
Phenylosazones crystallize readily (unlike sugars) and are useful derivatives.
The mechanism for osazones formation probably depends on a series of reaction on which `gt C == N -` behaves very much like `gt C == O` in giving a nitrogen version of an enol:
