Explanation
https://docs.splunk.com/Documentation/Splunk/8.1.0/SearchReference/ConversionFunctions#tostring.28X.2CY.

Reference: https://answers.splunk.com/answers/103/transaction-vs-stats-commands.html The stats command is faster and more efficient than the transaction command, especially in large environments. The stats command is used to calculate summary statistics on the events, such as count, sum, average, etc. The stats command can group events by one or more fields or by time buckets. The stats command does not create new events from groups of events, but rather creates new fields with statistical values. The transaction command is used to group events into transactions based on some common characteristics, such as fields, time, or both. The transaction command creates new events from groups of events that share one or more fields. The transaction command also creates some additional fields for each transaction, such as duration, eventcount, startime, etc. The transaction command is slower and more resource-intensive than the stats command because it has to process more data and create more events and fields.

Data model attributes are fields that are added to a dataset from search time field extractions, calculated fields, lookups, or aliases. They can be created within the data model editor or inherited from a parent dataset. They can be edited or removed unless they are required by the data model. They can be added to any type of dataset, not just root search datasets.ReferencesSee About data models, [Define data model attributes], and [Edit data model datasets] in the Splunk Documentation.

Explanation
The correct answer is D. eval.
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 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.
A calculated field is a shortcut for performing repetitive, long, or complex transformations using the eval command. The eval command is used to create or modify fields by using expressions. The eval command can perform mathematical, string, date and time, comparison, logical, and other operations on fields or values2.
For example, if you want to create a new field named total that is the sum of two fields named price and tax, you can use the eval command as follows:
| eval total=price+tax
However, if you want to use this new field in multiple searches, reports, or dashboards, you can create a calculated field instead of writing the eval command every time. To create a calculated field with Splunk Web, you need to go to Settings > Fields > Calculated Fields and enter the name of the new field (total), the name of the sourcetype (sales), and the eval expression (price+tax). This will create a calculated field named total that will be added to all events with the sourcetype sales at search time. You can then use the total field like any other extracted field without writing the eval expression1.
The other options are not correct because they are not related to calculated fields. These options are:
A: transaction: This command is used to group events that share some common values into a single record, called a transaction. A transaction can span multiple events and multiple sources, and can be useful for correlating events that are related but not contiguous3.
B: lookup: This command is used to enrich events with additional fields from an external source, such as a CSV file or a database. A lookup can add fields to events based on the values of existing fields, such as host, source, sourcetype, or any other extracted field.
C: stats: This command is used to calculate summary statistics on the fields in the search results, such as count, sum, average, etc. It can be used to group and aggregate data by one or more fields.
References:
About calculated fields
eval command overview
transaction command overview
[lookup command overview]
[stats command overview]