What are $X$ and $Y$ respectively in the following reaction sequence?

Consider the following reaction,where:
$(A)$ The change in the functional group and
$(B)$ The corresponding change in the hybridization from the starting material to the final product are:
$CH_3-C \equiv N \xrightarrow[(ii) \ HI + \text{Red } P]{(i) \ SnCl_2, HCl / H_3O^{+}}$

$3$-hydroxybutanal is formed when $(X)$ reacts with $(Y)$ in a dilute $(Z)$ solution. What are $X, Y$ and $Z$?

Consider the following reaction:
$H-CHO-(CH_2)_4-OH \xrightarrow{H^+} \text{Cyclic product}$
The above reaction is an example of:

Iodoform test is not given by

Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reaction :
$(P)$ Benzaldehyde
$(Q)$ $p-$Tolualdehyde
$(R)$ $p-$Nitrobenzaldehyde
$(S)$ Acetophenone