The correct answer is D. Search.
A workflow action is a knowledge object that enables a variety of interactions between fields in events and
other web resources.Workflow actions can create HTML links, generate HTTP POST requests, or launch
secondary searches based on field values1.
There are three types of workflow actions that can be set up using Splunk Web: GET, POST, and Search2.
GET workflow actions create typical HTML links to do things like perform Google searches on specific
values or run domain name queries against external WHOIS databases2.
POST workflow actions generate an HTTP POST request to a specified URI.This action type enables
you to do things like creating entries in external issue management systems using a set of relevant field
values2.
Search workflow actions launch secondary searches that use specific field values from an event, such as
a search that looks for the occurrence of specific combinations of ipaddress and http_status field values
in your index over a specific time range2.
Therefore, the workflow action type that performs a secondary search is Search.
References:
Splexicon:Workflowaction
About workflow actions in Splunk Web