Which of the following numbers will replace the question mark ($?$) in the given series?
31, 33, 37, 43, 51, ?

Let's find the difference between consecutive terms in the series:
* $33-31=2$
* $37-33=4$
* $43-37=6$
* $51-43=8$

The differences form a sequence of consecutive even integers: $2,4,6,8,\dots$
Following this logic, the next difference must be $10$.

Therefore, the missing number is:

$$\text{Missing number}=51+10=61$$