Which of the followings are nucleophile?

(a) ${\mathrm{HS}}^{-}$

(b) ${\mathrm{BF}}_3$

(c) ${\mathrm{C}}_2{\mathrm{H}}_5-\mathrm{OH}$

(d) ${\left({\mathrm{CH}}_3\right)}_3\ddot{\mathrm{N}}$

(e) $:{\mathrm{CH}}_2$

Arrange the given species in decreasing order of their nucleophilicity:

$\begin{array}{cccc}{\mathrm{H}}_2\mathrm{O},& {\mathrm{Cl}}^{⊝},& {\mathrm{Br}}^{⊝},& {\mathrm{I}}^{⊝}\end{array}$

Arrange the given species in decreasing order of their nucleophilicity:

Which of the following reactions are substitution reaction?

The following reaction is an example of