What is dehydrohalogenation in haloalkanes?

The major product of the following reaction is:

The major product in the following reaction is:

The major product of the following reaction is:

Heating of 2-chloro-1-phenylbutane with $\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{K}/\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}$ gives $\mathrm{X}$ as the major product. Reaction $\mathrm{X}$ with $\mathrm{H}\mathrm{g}{\left(\mathrm{O}\mathrm{A}\mathrm{c}\right)}_2/{\mathrm{H}}_2\mathrm{O}$ followed by $\mathrm{N}\mathrm{a}\mathrm{B}{\mathrm{H}}_4$ gives $\mathrm{Y}$ as the major product. $\mathrm{Y}$ is:

The major product of the following reaction is: