https://docs.splunk.com/Documentation/Splunk/8.1.0/SearchReference/ConversionFunctions#tostring.28X.2CY.29

The eval command 'if' function requires the following three arguments (in order): boolean expression, result if true, result if false. The eval command is a search command that allows you to create new fields or modify existing fields by performing calculations or transformations on them. The eval command can use various functions to perform different operations on fields. The 'if' function is one of the functions that can be used with the eval command to perform conditional evaluations on fields. The 'if' function takes three arguments:
a boolean expression that evaluates to true or false, a result that will be returned if the boolean expression is true, and a result that will be returned if the boolean expression is false. The 'if' function returns one of the two results based on the evaluation of the boolean expression.

"Calculated fields can reference all types of field extractions and field aliasing, but they cannot reference lookups, event types, or tags."

Highlighted search terms indicate matching search results in Splunk, which means that they show which parts of your events match your search string2. For example, if you search for error OR fail, Splunk will highlight error or fail in your events to show which events match your search string2. Therefore, option D is correct, while options A, B and C are incorrect because they are not indicated by highlighted search terms.