The S_N2 reaction involves back-side attack and therefore results in a "Walden Inversion." For which one of the substrates shown would you be able to demonstrate that such back-side attack with "Walden Inversion" has in fact occurred?

The total number of compounds amongst the following, that can be classified as vicinal dihalides will be  

How many number of monochlorinated products are obtained by $3 -$ethylpentane?

How many of the following alkyl halides will readily undergo ${\mathrm{S}}_{\mathrm{N}}1$ reaction mechanism?

Number of chiral carbon atoms in the compound $\mathrm{W},\mathrm{X},\mathrm{Y}$ and $\mathrm{Z}$ respectively would be