Solution:

The answer is (B). First, note that the sum of the numbers from $1$ to $2024$ is a multiple of $5$. This can be observed directly by knowing that the sum of the numbers from $1$ to $n$ is $\frac{n(n+1)}{2}$; alternatively, one can note that each of the following numbers is a multiple of $5$: $1+2024$, $2+2023, \ldots, 1012+1013$, and therefore so is the sum of these. Since the sum computed by Vittorio is a multiple of $5$, the missing number must also be a multiple of $5$. The answer is therefore $100$.