ANSWER
[tex]P(\text{even)}=\frac{1}{2}[/tex]EXPLANATION
The bag contains balls labeled 1 through 8.
From 1 to 8, there are 4 even numbers and 4 odd numbers.
Therefore, the probability of picking an even number is the number of even numbers divided by the total number of balls.
That is:
[tex]\begin{gathered} P(\text{even)}=\frac{4}{8} \\ P(\text{even)}=\frac{1}{2} \end{gathered}[/tex]That is the answer.