The correct answer is B. Use the field values in an HTTP error event to create a new ticket in an external system.
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 example that would use a POST workflow action is B. Use the field values in an HTTP error event to create a new ticket in an external system. This example requires sending an HTTP POST request to the URI of the external system with the field values from the event as arguments.
The other examples would use different types of workflow actions. These examples are:
A) Perform an external IP lookup based on a domain value found in events: This example would use a GET workflow action to create a link to an external IP lookup service with the domain value as a parameter.
C) Launch secondary Splunk searches that use one or more field values from selected events: This example would use a Search workflow action to run another Splunk search with the field values from the event as search terms.
D) Open a web browser to look up an HTTP status code: This example would also use a GET workflow action to create a link to a web page that explains the meaning of the HTTP status code.
Reference:
Splexicon:Workflowaction
About workflow actions in Splunk Web

Reference:https://docs.splunk.com/Documentation/Splunk/8.0.3/Search/Abouttransactions

A chart is a graphical representation of your search results that shows the relationship between two or more
fields2. You can display a chart in stack mode by changing the Stack Mode option in the Format menu2. Stack
mode allows you to stack multiple series on top of each other in a chart to show the cumulative values of each
series2. Therefore, option C is correct, while options A, B and D are incorrect because they are not ways to
display a chart in stack mode.

Reference:
https://answers.splunk.com/answers/574643/field-showing-an-additional-and-not-visible-value-1.html To use a macro in a search, you must enclose the macro name and any arguments in single quotation marks1.
For example, 'my_macro(arg1,arg2)' is a valid way to use a macro with two arguments. You can use macros anywhere in your search string where you would normally use a search command or expression1. Therefore, options A and C are valid searches that use macros, while options B and D are invalid because they do not enclose the macros in single quotation marks.