Recall the nature of oxides

Oxides are generally classified as:

• Acidic oxides: usually oxides of non-metals, e.g. $SO_3$, $SiO_2$
• Basic oxides: usually oxides of metals, especially alkali and alkaline earth metals, e.g. $CaO$
• Amphoteric oxides: show both acidic and basic behavior, e.g. $Al_2O_3$

Check each option

A. $SO_3$

• Sulfur is a non-metal.
• $SO_3$ is an acidic oxide because it forms sulfuric acid with water: $SO_3 + H_2O \rightarrow H_2SO_4$
• So, this is not a basic oxide.

B. $SiO_2$

• $SiO_2$ is generally an acidic oxide.
• It reacts with bases to form silicates.
• So, this is not a basic oxide.

C. $CaO$

• Calcium is an alkaline earth metal.
• $CaO$ is a typical basic oxide.
• It reacts with water to form a base: $CaO + H_2O \rightarrow Ca(OH)_2$
• Hence, this is the basic oxide.

D. $Al_2O_3$

• $Al_2O_3$ is amphoteric.
• It reacts with both acids and bases.
• So, it is not purely basic.

Final conclusion

The basic oxide among the given options is: $\boxed{CaO}$

Comparison with stored correct answer

• Derived answer: C
• Stored correct answer: C
• They match.