A Web application firewall (WAF) is able to detect and _____ new unknown attacks by watching for unfamiliar patterns in the traffic data.
While similar, the verbs avoid and prevent are not the same idea.
To avoid is to not go near something that is currently happening. To prevent is to stop something from existing or happening altogether.
For the sentence above, a WAF can prevent an attack from happening, but it cannot avoid an attack that is currently happening.