Which among the following compounds is a hemiacetal?

The reactant is $3$-methylpentanal. The reaction with $NaCN$ followed by $H_2SO_4$ (cyanohydrin formation) is shown below:
$CH_3CH_2CH(CH_3)CH_2CHO \xrightarrow{NaCN/H^+} CH_3CH_2CH(CH_3)CH_2CH(OH)CN$
Products of the reaction are:

Which of the following reactions converts the carbonyl group of aldehydes and ketones to a methylene group on treatment with zinc amalgam and concentrated $HCl$?

What are $X$ and $Y$ respectively in the following reactions?
$C_6H_5COOH \xrightarrow{X} C_6H_5CHO$
$C_6H_5CHO \xrightarrow[OH^-, 293 \ K]{C_6H_5COOH} Y \text{ (Major product)}$

What product is formed by the self-condensation of acetaldehyde in the presence of a dilute alkali?

For the conversion of cyclohexanone to $1-$hydroxy$-1-$methylcyclohexan$-2-$one,the correct order of reagents used is:
$(1)$ $CH_3MgBr / H^+$
$(2)$ $H^+ / \Delta$
$(3)$ $KMnO_4$ (cold dil.)
$(4)$ $CrO_3$