Step 1: Method:
Assign oxidation numbers using the rule that the algebraic sum equals the charge on the species, then compare.
Step 2: Compute each value:
Nitric acid: let N be $x$. $x + 1 - 6 = 0$, so $x = +5$.
Nitrous oxide: $2x - 2 = 0$, so $x = +1$.
Step 3: Compare:
$5 - 1 = 4$. Nitrogen is reduced by 4 units, as expected since nitric acid acts as an oxidising agent.
Step 4: Option check:
Only option (C) equals 4.
Final Answer:
The difference in oxidation number is 4, option (C).
\[ \boxed{4} \]