Which is the correct way to doin prefix pro proper adjective European?

The correct way to form the prefix pro- with the proper adjective European is pro-European.

Here's why:

* Pro- is a prefix meaning "in favor of" or "supporting."

* European is a proper adjective, meaning it refers to a specific place or people.

When combining prefixes with proper adjectives, we generally don't change the spelling of the adjective. This is to avoid confusion and maintain consistency.

Therefore, "pro-European" is the correct and standard way to write it.