$\underset{\begin{smallmatrix} 
 \text{Elimination} \\ 
 \,\,\,\,\,\text{reaction} 
\end{smallmatrix}}{\mathop{\xrightarrow{{{C}_{2}}{{H}_{5}}OH}}}\,\underset{\text{2-Pentene}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}{\mathop{C{{H}_{3}}-CH=CH-C{{H}_{2}}-C{{H}_{3}}}}\,$

When alkyl halide reacts with alc. $ KOH$ then it favours elimination reaction (Dehydrohalogenation). Since, trans pentene $-2$  is more symmetrical than cis isomers. Hence, it is main product.