Step 1: Recall the acid-base trend of oxides.
Metal oxides are generally basic, non-metal oxides are generally acidic, but some non-metal oxides in low oxidation states are neutral. We test each option against this.
Step 2: Examine N$_2$O$_5$.
Nitrogen is a non-metal in a high $+5$ state. Its oxide reacts with water to give an acid: $\text{N}_2\text{O}_5 + \text{H}_2\text{O} \rightarrow 2\text{HNO}_3$. So it is acidic.
Step 3: Examine NO.
Nitric oxide is a classic neutral oxide; it does not behave as an acid or a base.
Step 4: Examine Na$_2$O.
Sodium is a strongly electropositive metal, and its oxide reacts with water to give a base: $\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}$. So it is basic.
Step 5: Examine CO.
Carbon monoxide is another well-known neutral oxide.
Step 6: Choose the acidic oxide.
Only N$_2$O$_5$ forms an acid with water, so it is the acidic oxide - option (1).
\[ \boxed{\text{Acidic oxide: N}_2\text{O}_5\ \text{(option 1)}} \]