Total number of acyclic and cyclic isomers possible for molecular formula $C_4H_6$ is

According to $CIP$ sequence rule,the correct arrangement in order of decreasing priority is :-

Compare the basic strengths of the given compounds:

When isomers have the same structural formula but differ in the relative arrangement of atoms or groups,they are called ........

$P$ and $Q$ are isomers of dicarboxylic acid $C_4H_4O_4$. Both decolorize $Br_2/H_2O$. On heating,$P$ forms a cyclic anhydride.
Upon treatment with dilute alkaline $KMnO_4$,$P$ as well as $Q$ could produce one or more than one from $S$,$T$,and $U$.
$1.$ Compounds formed from $P$ and $Q$ are,respectively:
$(A)$ Optically active $S$ and optically active pair $(T, U)$
$(B)$ Optically inactive $S$ and optically inactive pair $(T, U)$
$(C)$ Optically active pair $(T, U)$ and optically active $S$
$(D)$ Optically inactive pair $(T, U)$ and optically inactive $S$
$2.$ In the following reaction sequences,$V$ and $W$ are respectively:
Give the answer for question $1$ and $2$.

Determine the correct priority order for the following compound to assign the $R$ or $S$ configuration.
$\begin{array}{c} CHO \\ | \\ H-C-OH \\ | \\ CH_2OH \end{array}$