Extent of dehydration by concentrated sulphuric acid
The most stable Alkene is the most highly substituted Alkene. (d) gives a tri-substituted alkene, all others give the less disubstituted alkenes. Most stable Alkene (d) therefore more of it is formed! (Edited thanks to Bon for highlighting my error)

Also a factor to keep in mind is conjugation. (a), (b) and (d) will have conjugated double-bonds, while (c) will not. This conjugation also adds stability to the system. But the major stability contributor is the higher substituted alkene in (d).