Answer: The concentration decreases.
Explanation: To dilute an acid means to add water to it. If you add water to an acid (or anything, really), the acid would have more space to spread out, and its concentration decreases. Think of it like this: if you add a drop of red food coloring into a bottle and add a very small amount of water (10mL), the color would be very red. If you add a large amount of water (3L), however, you would barely notice the red color.