Which is the preferred testing nowadays for scripting?

Answer: Option B

In the past, the Navigator object was commonly used by scripts to determine if they were running in Internet Explorer or Netscape. This “browser-sniffing” approach is problematic because it requires constant tweaking as new browsers and new versions of existing browsers are introduced. Today, feature testing is preferred: rather than making assumptions about particular browser versions and their features, you simply test for the feature (i.e., the method or property) you need.