Select the number that will replace the question mark (?) in the following number series.
1, 1, 2, 3, 5, 8, 13, ?

The correct answer is 21.

Step-by-step explanation:

The given number series is:

$1,1,2,3,5,8,13,?$

This sequence follows a pattern where starting from the third term, each term is the sum of the two preceding terms (the Fibonacci sequence pattern).

Let us analyze the relationships between successive terms:

$1+1=2$

$1+2=3$

$2+3=5$

$3+5=8$

$5+8=13$

Following this established pattern, the term that replaces the question mark (?) is obtained by adding the last two terms, 8 and 13:

$8+13=21$

Therefore, the number that will replace the question mark is 21.