Dehydration of alcohols to alkenes is catalysed by concentrated sulphuric acid ($H_2SO_4$) or phosphoric acid ($H_3PO_4$). The reaction typically proceeds via an E1 mechanism involving the formation of a carbocation intermediate.

1. Protonation: The alcohol accepts a proton to form a protonated alcohol.
2. Carbocation Formation: Water leaves, generating a carbocation. This is the slow, rate-determining step.
3. Rearrangement: If the carbocation is secondary ($2^\circ$) or primary ($1^\circ$) and can rearrange to a more stable tertiary ($3^\circ$) carbocation via a 1,2-hydride or 1,2-methyl shift, this rearrangement will occur.
4. Elimination: A proton is removed from a $\beta$-carbon to form a double bond. According to Saytzeff's (Zaitsev's) rule, the major product is the most highly substituted alkene (the one with the greater number of alkyl groups attached to the double bond carbon atoms).

Any alkene that cannot be formed via the initial carbocation or its rearranged forms (following the Saytzeff or Hofmann elimination pathways) would be the correct answer for 'not a product'.