Which of the following summation notations represents this series?
3 + 8 + 13 + + 23

First step in such tasks, is to find what is between each number.

8-3 = 5
13-8 = 5

13-23 = 10

Based on only the first 2, and the last one as a backup we can assume that it jumps with 5 each time, starting on 3. Hence the open spot should be:
18.

The correct answer is 18. :)