A workflow action is a link that appears when you click an event field value in your search results1. A workflow action can open a web page or run another search based on the field value1. There are two types of workflow actions: GET and POST1. A GET workflow action appends the field value to the end of a URI and opens it in a web browser1. A POST workflow action sends the field value as part of an HTTP request to a web server1. You can configure a workflow action to open a web page in either the same window or a new window1. Therefore, option D is correct, while options A, B and C are incorrect.

https://docs.splunk.com/Documentation/Splunk/8.1.0/SearchReference/ConversionFunctions#tostring.28X.2CY.
The tostring function in the eval command converts a numeric value to a string value. It can take an optional second argument that specifies the format of the string value. Some of the possible formats are:
* hex: converts the numeric value to a hexadecimal string.
* commas: adds commas to separate thousands in the numeric value.
* duration: converts the numeric value to a human-readable duration string, such as "2h 3m 4s".
Therefore, the formats A, B, and D can be used with the tostring function.

Explanation

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