Find the Missing Number: 12,399 < □ < 13,001

Let's solve the problem by examining each possible answer choice to determine which one fits within the range $12,399 < \Box < 13,001$.

• Choice 1: $12,100$
  $12,100$ is less than $12,399$, so it does not satisfy $12,399 < 12,100$.
• Choice 2: $13,100$
  $13,100$ is greater than $13,001$, so it does not satisfy $13,100 < 13,001$.
• Choice 3: $12,850$
  $12,399 < 12,850$ and $12,850 < 13,001$. This choice satisfies both conditions.
• Choice 4: $12,300$
  $12,300$ is less than $12,399$, so it does not satisfy $12,399 < 12,300$.

Therefore, the number that fits between 12,399 and 13,001 is $12,850$.