Explanation:

In Salesforce B2C Commerce, if a developer creates a new cartridge to implement custom functionality (controllers, scripts, templates, etc.), the only way to activate it is to include it in the cartridge path of the Storefront site that will use it.

🔹 Steps to Apply Cartridge Changes:
Go to:
Administration → Sites → Manage Sites → [Your Site] → Settings
Edit the cartridge path and prepend your new cartridge:
Example:
app_custom_mycartridge:int_custom:int_paypal:app_storefront_base
Save changes

❌ Why the Other Options Are Incorrect:

A. Add the new cartridge to the Business Manager site
❌ That’s only relevant for cartridges that modify Business Manager modules, not storefront behavior
B. Rebuild site indexes
❌ Indexes affect search/catalog visibility, not cartridge code activation

🧠 Tip:
Cartridge order matters:
The first cartridge found in the path with a matching file takes precedence.
Always place your custom cartridges before base ones like app_storefront_base if you intend to override or extend functionality.

🔹 Reference:
📘 Salesforce B2C Commerce Documentation
Reference:
Salesforce B2C Commerce – Add a Cartridge to a Site

Quote:
"To use functionality from a cartridge in a site, the cartridge must be added to the cartridge path for that site. The order in which cartridges appear in the path determines which resources are used when multiple cartridges contain the same resource."