Reference: https://docs.splunk.com/Documentation/Splunk/8.0.3/Knowledge/definecalcfields Calculated fields are fields that are created by performing calculations on existing fields using the eval command. Calculated fields can be used in the search bar to filter and transform events based on the calculated values. Calculated fields can also be based on an extracted field, which is a field that is extracted from raw data using various methods, such as regex, delimiters, lookups, etc. Calculated fields are not shortcuts for performing calculations using the eval command, but rather results of performing calculations using the eval command. Calculated fields can be applied to any field in Splunk, not only host and sourcetype.
Therefore, statements A, B, and D are true about calculated fields.

https://docs.splunk.com/Documentation/Splunk/8.1.0/SearchReference/Chart
https://docs.splunk.com/Documentation/Splunk/8.1.0/SearchReference/Stats

The fillnull command can be used to replace null values in specific fields. The correct equivalent expression for the given eval statement would involve using fillnull twice, once for field1 to replace null values with 0, and once for field2 to replace null values with "NO-VALUE".
References:
Splunk Docs - fillnull command

https://docs.splunk.com/Documentation/Splunk/8.0.2/Search/Abouttransactions In other cases, it's usually better to use the stats command, which performs more efficiently, especially in a distributed environment. Often there is a unique ID in the events and stats can be used.