Explanation:

After creating a search refinement definition (to allow filtering by a new product attribute on the Storefront), you must ensure that:

The attribute contains actual data in the product catalog
The attribute is indexed in the search index
This makes it available to be used for refinements (faceted search) on the storefront

Even if you create the refinement configuration in Business Manager, it won’t work unless the attribute is indexed and populated.

🔧 Steps to Achieve Filtering on a New Attribute:
✅ Create the attribute in the product system object (if not already present).
✅ Populate the attribute with values for the appropriate products.
✅ Configure it as searchable and refinable in Business Manager:
Go to Merchant Tools → Search → Searchable Attributes
Set:
Searchable = true (if you want to search by it)
Use in Search Refinements = true (to enable filter)
✅ Rebuild the search index so the changes take effect.
✅ Verify in storefront: You should see the new refinement appear in the product listing page sidebar.

❌ Why the Other Options Are Incorrect:
B. Set the attribute as Searchable
This is necessary only if you want the attribute to be searchable, but not sufficient alone to make it refinable. Also, indexing is still required.
C. Change productsearchrefinebar.isml
The SFRA template already supports dynamic refinements. You don’t need to change the ISML unless you’re customizing layout or display.

📘 Reference:
Searchable Attributes & Refinements
Indexing Product Attributes for Search