Which is the best technique to test a script for cross-browser compatibility?

Which is the best technique to test a script for cross-browser compatibility?

A.
Use the W3C site’s code validator to test the script.

B.
Test the script in various versions of the same browser.

C.
Test the script in various browsers, including various versions of the same browsers.

D.
Determine the most frequently used browser for your target audience and then test the script in that browser.