Explanation:

The most efficient and scalable method that uses the product search index rather than searching the database is C: ProductSearchModel().getProductSearchHits(). This method leverages the B2C Commerce search index, which is optimized for performance and scalability, to retrieve product search results, making it ideal for large-scale queries compared to direct database searches.

Reasoning:
Search Index vs. Database: B2C Commerce uses a search index (maintained via Business Manager > Merchant Tools > Search > Indexing) to optimize product searches, reducing the load on the database. Methods relying on the index are more efficient for large catalogs or frequent queries.

A. ProductIndexModel.getOrderableProductsOnly()
Incorrect. There is no ProductIndexModel or getOrderableProductsOnly() method in the B2C Commerce Script API. This appears to be a distractor, possibly a misinterpretation of index-related functionality.

B. ProductAvailabilityModel.isOrderable()
Incorrect. The ProductAvailabilityModel.isOrderable() method checks a product’s availability based on inventory data, querying the database directly rather than using the search index. It’s not designed for scalable product searches.

C. ProductSearchModel().getProductSearchHits()
Correct. The ProductSearchModel class (from dw.catalog.ProductSearchModel) uses the search index to retrieve a list of ProductSearchHit objects matching a query. This method is optimized for performance, supporting filters, sorting, and pagination, making it efficient and scalable for storefront search or category pages.
Example:
var ProductSearchModel = require('dw/catalog/ProductSearchModel');
var searchModel = new ProductSearchModel();
searchModel.setCategoryID('categoryID');
searchModel.search();
var hits = searchModel.getProductSearchHits();

D. ProductVariantModel.getVariants()
Incorrect. The ProductVariantModel.getVariants() method retrieves variant products for a master product, querying the database or product catalog directly. It does not utilize the search index, making it less efficient for large-scale searches.

Implementation Note:
The getProductSearchHits() method returns an iterator of ProductSearchHit objects, which include product IDs and relevance scores, optimized for search performance. It’s commonly used in controllers or scripts to power storefront search results.

Reference:
Salesforce B2C Commerce Documentation: ProductSearchModel API details index-based searching.
Trailhead Module: “B2C Commerce Developer” module on “Search and Navigation” covers search index usage.

Exam Tip:
For the Salesforce B2C-Commerce-Developer exam, identify ProductSearchModel().getProductSearchHits() as the index-based method, avoiding distractors with incorrect class names or database-dependent methods. Note typos (e.g., ProducrlndexModel, ProduccAvailabiliryModel) as common exam tricks.