Among the following, basic oxide is

Let us classify each oxide:

$$SO_3$$ (Sulfur trioxide): Sulfur is a non-metal. $$SO_3$$ reacts with water to form sulfuric acid: $$SO_3 + H_2O \rightarrow H_2SO_4$$. Therefore, $$SO_3$$ is an acidic oxide.

$$SiO_2$$ (Silicon dioxide): Silicon is a metalloid. $$SiO_2$$ reacts with bases (e.g., $$NaOH$$) but not with acids. Therefore, $$SiO_2$$ is an acidic oxide.

$$CaO$$ (Calcium oxide): Calcium is an alkaline earth metal. $$CaO$$ reacts with water to form calcium hydroxide: $$CaO + H_2O \rightarrow Ca(OH)_2$$, which is a strong base. Therefore, $$CaO$$ is a basic oxide.

$$Al_2O_3$$ (Aluminium oxide) reacts with both acids and bases. Therefore, $$Al_2O_3$$ is an amphoteric oxide.

Hence, the correct answer is Option C (CaO).