The correct answer is 16.

Solution:

Considering the given number pattern.

Here, the numbers are increasing.
So, to find the missing number, we first find the rule.

Note that:
1 + 5 = 6
6 + 5 = 11
11 + 5 = 16
16 + 5 = 21

So, the rule is "add 5".

Then, the missing number is 16.