Which of the following is most reactive towards nucleophilic substitution reaction by both ${\mathrm{S}}_{\mathrm{N}}1$ and ${\mathrm{S}}_{\mathrm{N}}2$ mechanisms?

Which describes the best stereochemical aspects of the following reaction?

What is the major elimination product of following alkyl bromide ?

What is the major product of the following reaction ?

What is the reactant $\mathrm{X}$ in the above reaction?

If 

what will be the major product of the following reaction

 major product.