Final answer:
The correct format for a custom campaign parameter added to a URL is /?utm_campaign=fallsale.
Step-by-step explanation:
The correct format for a custom campaign parameter added to a URL is option b. /?utm_campaign=fallsale. In this format, the parameter name starts with 'utm_' followed by the desired campaign name, which is then assigned using the equals sign ( = ) and enclosed in single or double quotations. It is important to use underscores instead of spaces and avoid any unnecessary spaces within the parameter.