Select the number from among the given options that can replace the question mark ($?$) in the following number series.
$$1,1,2,3,5,8,13,?$$

The given series is the famous Fibonacci sequence, where each subsequent term is obtained by adding the two preceding terms together:
* $1+1=2$
* $1+2=3$
* $2+3=5$
* $3+5=8$
* $5+8=13$

Following this logic, the next term will be:

* $8+13=21$

Thus, the missing number is $21$.