The best leaving group, in this case, is the mesylate.
Work Step by Step
1. Identify the possibles leaving groups: - $EtO^-$ ($CH_3CH_2O^-$) - $Mesylate$ ($CH_3O_3S^-$) 2. Determine which one is better. - In this section, we've learned that sulfonate ions, like the mesylate, are good leaving groups (page 219). - And we have also learned that, the $CH_3CH_2O^-$ is a bad leaving group (page 218). Therefore, this mesylate is better.