What is Fehling solution?

The reaction shown below is:

Which of the following compounds will not react with phenylhydrazine?

Compound $(A)$ with molecular formula $C_6H_{12}O_3$ gives a positive iodoform test and a negative Tollens' test. Upon hydrolysis with a drop of $H_2SO_4$,it gives a product that shows a positive Tollens' test. Compound $(A)$ is:

$CH_3-CO-CH_3$ $\xrightarrow{I_2/NaOH} \underset{\text{yellow compound}}{(A)} + (B)$ $\xrightarrow{Ag - \text{powder}, \Delta} (C)$ $\xrightarrow{Dil. H_2SO_4/HgSO_4} (D)$
$A, C$ and $D$ are :-

Identify the products $(A)$ and $(B)$ from the given structures $P, Q, R$.