Which of the following aldehydes is most reactive $?$

Arrange the following compounds in decreasing order of reactivity towards nucleophilic addition reactions:
$(I) \ HCHO$,$(II) \ CH_3CHO$,$(III) \ CH_3COCH_3$,$(IV) \ CH_3CH_2COCH_2CH_3$

Which of the following cannot be obtained by the reduction of an aldehyde or a ketone using $NaBH_4$ or $LiAlH_4$?

Which among the following compounds give a yellow solid when reacted with $NaOI/NaOH$? Choose the correct answer from the options given below $:$
$(A)$ $CH_3-CH(OH)-C_2H_5$
$(B)$ $CH_3-CH_2-CH_2-OH$
$(C)$ $CH_3-CO-C_2H_5$
$(D)$ $CH_3-COOH$
$(E)$ $CH_3-CH_2-CHO$

Statement $I$: Aldehyde on reaction with $HCN$ gives Cyanohydrin.
Statement $II$: Cyanohydrin is a compound which consists of hydroxy and cyano groups on the same carbon.
Choose the correct answer from the following with reference to above statements.

In the following reaction, the major product $(H)$ is