An halide C5H11Br  on treatments with Alc. KOH given 2–Pentene only the halide will be

1-Bromo pentane (option 1) on dehydro halogenation gives 1-pentene.2-Bromo pentane (option 2) on dehydro halogenation gives 1-pentene and 2-pentenene2-Bromo-3-methyl  pentane (option 4) on dehydro halogenation gives 2-methyl-2-butene .Therefore option 3 is correct.since it  on treatments with Alc. KOH gives 2–Pentene only.