Reference: https://docs.splunk.com/Splexicon:Calculatedfield

Reference:
https://docs.splunk.com/Documentation/Splunk/8.0.3/Knowledge/TagandaliasfieldvaluesinSplunkWeb

The rex command allows you to extract fields from events using regular expressions. You can use the rex command to specify a named group that matches the port number in the event. For example:
rex "\+\+\+\+port (?<port>\d+)"
This will create a field called port with the value 54 for the event.
The delimiter method is not suitable for this event because there is no consistent delimiter between the fields.
The regular expression method is not a valid option for the Field Extractor tool. The Field Extractor tool can extract regular expressions, but it is not a method by itself.
Reference: 1 Splunk Core Certified Power User | Splunk

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.
Reference:
Splexicon:Workflowaction
About workflow actions in Splunk Web