Final answer:
To narrow a search to a specified category using a category number, one would use the Filter key. This option applies search parameters to limit results to relevant items in the category.
Step-by-step explanation:
To narrow your search to a defined category using a category number, the key you would typically use is Filter. The filter function allows you to apply specific parameters to your search, thereby limiting the results to the most relevant items within the chosen category. Unlike 'Search' which initiates a new search, 'Sort' which arranges existing search results, or 'Index' which helps navigate through a list, 'Filter' specifically narrows down a search to a subset based on criteria or categories.