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.

Reference:
http://hilllaneconsulting.co.uk/blog/?p=640

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