What is the proper form of address for a college professor?

The proper form of address for a college professor is either "Professor" or "Dr.", followed by the professor's last name.

For example: Professor Smith or Dr. Smith.

If you are not sure of the professor's title, you may also simply use the prefix "Mr." or "Ms./Mrs." followed by the professor's last name. For example: Mr. Smith or Ms. Smith.

However, it is always best to address a college professor by their proper title, as this shows respect and courtesy.