In Splunk, the transaction command is used to group events that share common characteristics into a single transaction1. By default, the transaction command groups all matching events into a single transaction1.
However, you can use the maxspan option to limit the time span of the transactions1. If the time span between the first and last event in a transaction exceeds the maxspan value, the transaction command will start a new transaction1.
Therefore, if the maxspan option is not included in the search, the transaction command might produce multiple transactions instead of one if the time span between the first and last event in a transaction exceeds the default maxspan value1.
Here is an example of how you can use the maxspan option in a search:
index=main sourcetype=access_combined | transaction someuniqefield maxspan=1h In this search, the transaction command groups events that share the same someuniqefield value 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, the transaction command will start a new transaction1.

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The correct answers are B. duration and D. transaction id.
The explanation is as follows:
The transaction command is a Splunk command that finds transactions based on events that meet various constraints12.
Transactions are made up of the raw text (the _raw field) of each member, the time and date fields of the earliest member, as well as the union of all other fields of each member12.
The transaction command adds some fields to the raw events that are part of the transaction123. These fields are:
duration: The difference, in seconds, between the timestamps for the first and last events in the transaction123.
eventcount: The number of events in the transaction123.
transaction_id: A unique identifier for each transaction3. This field is useful for filtering or joining transactions3.
Therefore, the fields that the transaction command adds to the raw events are duration and transaction_id, which are options B and D in your question.

The correct answer is B. Root event dataset. This is because root event datasets are defined by a constraint that filters out events that are not relevant to the dataset. A constraint for a root event dataset is a simple search that returns a fairly wide range of data, such as sourcetype=access_combined. Without a constraint, a root event dataset would include all the events in the index, which is not useful for data modeling. You can learn more about how to design data models and add root event datasets from the Splunk documentation1.
The other options are incorrect because root transaction datasets and root search datasets have different ways of defining their datasets, such as transaction definitions or complex searches, and root child datasets are not a valid type of root dataset.

Reference: https://docs.splunk.com/Splexicon:Calculatedfield
The eval command is used to create new fields or modify existing fields based on an expression2. The output of an eval expression is a calculated field, which is a field that you create based on the value of another field or fields2. You can use calculated fields to enrich your data with additional information or to transform your data into a more useful format2. Therefore, option B is correct, while options A, C and D are incorrect because they are not names of knowledge objects that represent the output of an eval expression.

The correct answer is B. eventtype=web_errors.
An event type is a way to categorize events based on a search. An event type assigns a label to events that match a specific search criteria. Event types can be used to filter and group events, create alerts, or generate reports1.
To search for events that have a specific event type, you need to use the eventtype field with the name of the event type as the value. The syntax for this is:
eventtype=<event_type_name>
For example, if you want to search for events that have the event type web_errors, you can use the following syntax:
eventtype=web_errors
This will return only the events that match the search criteria defined by the web_errors event type.
The other options are not correct because they use different syntax or fields that are not related to event types.
These options are:
* A. tag=web_errors: This option uses the tag field, which is a way to add descriptive keywords to events based on field values. Tags are different from event types, although they can be used together. Tags can be used to filter and group events by common characteristics2.
* C. eventtype "web errors": This option uses quotation marks around the event type name, which is not valid syntax for the eventtype field. Quotation marks are used to enclose phrases or exact matches in a search3.
* D. eventtype (web_errors): This option uses parentheses around the event type name, which is also not valid syntax for the eventtype field. Parentheses are used to group expressions or terms in a search3.
References:
* About event types
* About tags
* Search command cheatsheet