Which search will return the 15 least common field values for the dest_ip field?

The correct choice utilizes the rare command effectively to discover the least common values for the specified field, which in this case is dest_ip. When applied to a dataset, the rare command identifies the infrequently occurring values within a specified field.

Specifically, the limit parameter allows you to define how many rare values you want to retrieve. By setting limit=15, the search will return exactly 15 values from the dest_ip field that appear the least frequently across the given dataset where the sourcetype is defined as firewall.

This choice specifically addresses the requirement to find the least common values, ensuring that the output is tailored to the user's need for a specific number of rare occurrences (in this case, 15). The other options fail to use the correct parameter for limiting the number of results effectively. For example, while parameters like num, last, and count might suggest some form of limiting or counting, only limit is valid in the context of specifying the number of results directly related to the rare command's output.