Which of the following compounds when treated with ammoniacal silver nitrate exhibits silver mirror test?

In the following compounds,the ones that give positive iodoform test are:
$(I)$ $CH_3CH_2COCH_2CH_3$
$(II)$ $CH_3CH(OH)CH_3$
$(III)$ $1$-indanone
$(IV)$ $CH_3CH_2COCH_3$
$(V)$ $PhCOPh$
$(VI)$ $PhCOCH_3$

The major product '$A$' formed in the above reaction is:

The addition of $HCN$ to carbonyl compounds is an example of

The above conversion can be carried out by,

Which of the following aldehydes is most reactive towards nucleophilic addition reactions?