Which of the following numbers logically replaces the question mark to complete the sequence below?

21, 34, 55, ?

The correct option is 89.

To find the missing number in the sequence 21, 34, 55, ?, let us observe the pattern formed by the numbers:

This sequence follows the famous Fibonacci pattern, where each term after the first few terms is the sum of the two preceding terms.

Let us verify this relationship between the given terms:

$21+34=55$

Following the same logical pattern, the number that replaces the question mark (?) is obtained by adding the last two known numbers in the sequence (34 and 55):

$34+55=89$

Thus, the number that logically replaces the question mark is 89.