Final answer:
EtO- is the strongest base because the ethyl group helps better stabilize the negative charge through electron-donation, compared to the other options.
Step-by-step explanation:
The strongest base among the options given is EtO- because, as a general trend in base strength, the more delocalized the negative charge is, the stronger the base. Ethoxide ion (EtO-) contains an ethyl group (a carbon chain), which is electron-donating through the inductive effect and better stabilizes the negative charge compared to simple alkoxides like methoxide (OCH₃-). In the context of the other options, OH⁻ is a strong base but not as strong as some organic bases. Br⁻ is the conjugate base of a strong acid, which makes it a very weak base. In the case of the OCH₃-, it is similarly a methoxide ion, which is less stable than the ethoxide ion due to less steric hindrance around the oxygen atom, thus, less able to spread out the negative charge.