Which of the following halohydrins gives cyclohexene oxide readily when treated with aqueous NaOH?

The alkoxide formed by NaOH must attack the carbon bearing Cl from the back side, so O and Cl must be anti (diaxial in the chair). This is possible in the trans-2-chloro isomer. In the cis isomer the groups are always gauche and no anti arrangement is possible. The 3- and 4-chloro isomers would give strained bridged ethers.