Which of the following is the most basic oxide?

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1. **Analyze Periodic Trends:** Metallic oxides are generally basic, while non-metallic oxides are acidic. Within a group, basic character increases down the group as metallic character increases. Across a period, acidic character increases.
2. **Analyze $Al_2O_3$ (Aluminium Oxide):** Aluminium is amphoteric. $Al_2O_3$ reacts with both acids and bases [Class 12 Chemistry, Unit 4, Sec 4.4]. It is **amphoteric**.
3. **Analyze $Sb_2O_3$ (Antimony(III) Oxide):** Antimony is a metalloid (Group 15). Its oxide is **amphoteric**.
4. **Analyze $SeO_2$ (Selenium Dioxide):** Selenium is a non-metal (Group 16). Non-metallic oxides are acidic. $SeO_2$ is **acidic**.
5. **Analyze $Bi_2O_3$ (Bismuth(III) Oxide):** Bismuth is a metal (Group 15, below Antimony). As we move down Group 15, the metallic character increases ($N < P < As < Sb < Bi$). Consequently, the basic nature of their oxides increases. $Bi_2O_3$ is purely **basic**.
6. **Conclusion:** Among the given options, $Bi_2O_3$ is the only purely basic oxide.

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