Which of the following is a solid trimer of formaldehyde?

Which among the following is a possible product during cross aldol condensation between propanal (as electrophile) and butanal (as nucleophile)?

Cyclohexene on ozonolysis followed by reaction with zinc dust and water gives compound $E$. Compound $E$ on further treatment with aqueous $KOH$ yields compound $F$. Compound $F$ is

Consider the given reaction,the product $A$ is :

$A$ compound contains an $\alpha$-hydrogen atom and forms a $\beta$-hydroxy aldehyde in the presence of a dilute alkali. When this product is heated with a dilute acid,it yields unsaturated crotonaldehyde. Identify the compound.

Which of these reagents would accomplish the following reduction?
$NC-CH_2-CO-CH_2-CH=CH_2 \to NC-CH_2-CH(OH)-CH_2-CH=CH_2$