Which step is chain propagation step in the following mechanism?

$\left(\mathrm{i}\right)<mspace\; width="1em"></mspace>{\mathrm{Cl}}_2\stackrel{h\nu }{\to }{\mathrm{Cl}}^{\bullet }+{\mathrm{Cl}}^{\bullet }$
$\left(\mathrm{ii}\right)<mspace\; width="1em"></mspace>{\mathrm{Cl}}^{\bullet }+<mspace\; width="1em"></mspace>{\mathrm{CH}}_4\to \stackrel{\bullet }{{\mathrm{CH}}_3}+<mspace\; width="1em"></mspace>\mathrm{HCl}$
$\left(\mathrm{iii}\right){\mathrm{Cl}}^{\bullet }+{\mathrm{Cl}}^{\bullet }\stackrel{}{\to }{\mathrm{Cl}}_2$
$\left(\mathrm{iv}\right)\stackrel{\bullet }{{\mathrm{CH}}_3}+<mspace\; width="1em"></mspace>\mathrm{HCl}\to {\mathrm{CH}}_3\mathrm{Cl}$

1. (i)

2. (ii)

3. (iii)

4.  (iv)

The formation of cyanohydrin from a ketone is an example of:

(1) electrophilic addition

(2) nucleophilic addition

(3) nucleophilic substitution

(4) electrophilic substitution

In the given reaction,

[X] will be:

(1) 

(2) 

(3) 

(4) 

Which compound is least reactive for nucleophilic addition reaction?

(1) ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CHO}$

(2) ${\mathrm{C}}_6{\mathrm{H}}_5-\stackrel{\mathrm{O}}{\stackrel{\left|\right|}{\mathrm{C}}}-{\mathrm{CH}}_3$

(3) ${\mathrm{C}}_6{\mathrm{H}}_5-\stackrel{\mathrm{O}}{\stackrel{\left|\right|}{\mathrm{C}}}-{\mathrm{C}}_6{\mathrm{H}}_5$

(4) ${\mathrm{CH}}_3-\stackrel{\mathrm{O}}{\stackrel{\left|\right|}{\mathrm{C}}}-{\mathrm{CH}}_3$