Major product of the dehydration of alcohols
The mechanism of dehydration of 2° alcohol at high temperature (around 170°C-180°C) happens via $E_1$ mechanism. This means that the major product will be decided by the $\mathbf {Zaitsev}$ $\mathbf{rule}$, which says that major product will be the stables one. If you compare the three alkenes, you'll see that the middle compound you've drawn has the most hyperconjugative structures, and hence is the stablest. Therefore, that will be the major product.