The delimiter method in the Field Extractor should be used when fields in events are separated consistently by a specific character (such as a comma, tab, or pipe). This method simplifies extraction without requiring complex regular expressions.
Reference:
Splunk Power User Study Guide, Field Extraction Section
Splunk Docs: Using the Field Extractor
"Use the delimiter extraction method when your data fields are consistently separated by a specific delimiter character."

The correct answer is B. A field added by an automatic lookup.
A calculated field is a field that is added to events at search time by using an eval expression. A calculated field can use the values of two or more fields that are already present in the events to perform calculations. A calculated field can use any field as a source, as long as the field is extracted before the calculated field is defined1.
An automatic lookup is a way to enrich events with additional fields from an external source, such as a CSV file or a database. An automatic lookup can add fields to events based on the values of existing fields, such as host, source, sourcetype, or any other extracted field2. An automatic lookup is performed before the calculated fields are defined, so the fields added by the lookup can be used as sources for the calculated fields3.
Therefore, a calculated field can use a field added by an automatic lookup as a source.
References:
* About calculated fields
* About lookups
* Search time processing

The timechart command in Splunk requires the _time field to be present in the dataset because it uses time as the primary axis for charting data. The _time field represents the time of events and is essential for commands that generate visualizations based on time, such as timechart. This command groups the events into time intervals and performs statistical functions on those time intervals. Without the _time field, the timechart command will not function properly.
References:
Splunk Docs - timechart command

https://docs.splunk.com/Documentation/Splunk/latest/SearchReference/CommonEvalFunctions The eval command function tostring() is valid. The tostring() function converts a numeric value to a string value. For example, tostring(3.14) returns "3.14". The other functions are not valid eval command functions.