a
This is an example of elimination reaction. Alcoholic \(KOH\) shows \(E 2\) elimination reaction. The reactivity order for \(E 2\) elimination reaction depends upon the stability of the product formed as shown below :

\(3^{\circ}>2^{\circ}>1^{\circ}\)

The order of leaving group in \(E 2\) elimination reaction is shown below :

\(R - I > R - Br > R - Cl\)

Therefore, the correct order for reaction with alcoholic \(KOH\) is as follows :

\(a >  c > b > d\)