The correct answer is D. | eval notNULL = "" fillnull value=0 notNULL
Option A is incorrect because it is missing a comma between the "0" and the notNULL in the if function. The correct syntax for the if function is if (condition, true_value, false_value).
Option B is incorrect because it is missing the false_value argument in the if function. The correct syntax for the if function is if (condition, true_value, false_value).
Option C is incorrect because it uses the nullfill command, which only replaces null values, not empty strings. The nullfill command is equivalent to fillnull value=null.
Option D is correct because it uses the eval command to assign an empty string to the notNULL field, and then uses the fillnull command to replace the empty string with a zero. The fillnull command can replace any value with a specified replacement, not just null values.

Normalize your data for each of these fields using a combination of field aliases, field extractions, and lookups.
https://docs.splunk.com/Documentation/CIM/4.15.0/User/UsetheCIMtonormalizedataatsearchtime

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.