After manually editing a regular expression (regex) that was created using the Field Extractor (FX) UI, it is no longer possible to edit the field extraction in the FX UI. The FX UI is a tool that helps you extract fields from your data using delimiters or regular expressions. The FX UI can generate a regex for you based on your selection of sample values or you can enter your own regex in the FX UI. However, if you edit the regex manually in the props.conf file, the FX UI will not be able to recognize the changes and will not let you edit the field extraction in the FX UI anymore. You will have to use the props.conf file to make any further changes to the field extraction. Changes made manually cannot be reverted in the FX UI, as the FX UI does not keep track of the changes made in the props.conf file. It is possible to manually edit a regex that was created using the FX UI, as long as you do it in the props.conf file.
Therefore, only statement B is true about manually editing a regex.

https://docs.splunk.com/Documentation/Splunk/8.1.1/SearchReference/ConditionalFunctions

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.

Splunk alerts can be based on searches that run in real-time or on a regular schedule3. An alert is a way to monitor your data and get notified when certain conditions are met3. You can create an alert by specifying a search and a triggering condition3. You can also specify how often you want to run the search and how you want to receive the alert notifications3. You can run the alert search in real-time, which means that it continuously monitors your data as it streams into Splunk3. Alternatively, you can run the alert search on a regular schedule, which means that it runs at fixed intervals such as every hour or every day3. Therefore, options A and B are correct, while option C is incorrect because it is not a way to run an alert search.

The correct answer is B. Calculated fields can be chained together to create more complex fields.
Calculated fields are fields that are added to events at search time by using eval expressions. They can be used to perform calculations with the values of two or more fields already present in those events. Calculated fields can be defined with Splunk Web or in the props.conf file. They can be used in searches, reports, dashboards, and data models like any other extracted field1.
Calculated fields can also be chained together to create more complex fields. This means that you can use a calculated field as an input for another calculated field. For example, if you have a calculated field named total that sums up the values of two fields named price and tax, you can use the total field to create another calculated field named discount that applies a percentage discount to the total field. To do this, you need to define the discount field with an eval expression that references the total field, such as:
discount = total * 0.9
This will create a new field named discount that is equal to 90% of the total field value for each event2.
References:
* About calculated fields
* Chaining calculated fields