Explanation
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

In Splunk, thetransactioncommand is used to group events that share common characteristics into a single transaction1.By default, thetransactioncommand groups all matching events into a single transaction1.
However, you can use themaxspanoption to limit the time span of the transactions1.If the time span between the first and last event in a transaction exceeds themaxspanvalue, thetransactioncommand will start a new transaction1.
Therefore, if themaxspanoption is not included in the search, thetransactioncommand might produce multiple transactions instead of one if the time span between the first and last event in a transaction exceeds the defaultmaxspanvalue1.
Here is an example of how you can use themaxspanoption in a search:
index=main sourcetype=access_combined | transaction someuniqefield maxspan=1h In this search, thetransactioncommand groups events that share the samesomeuniqefieldvalue into a single transaction, but only if the time span between the first and last event in the transaction does not exceed 1 hour1.If the time span exceeds 1 hour, thetransactioncommand will start a new transaction1.

Reference:https://splunkonbigdata.com/2018/10/27/usage-of-splunk-eval-function-tostring/