Explanation
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.

Explanation
Field aliases are alternative names for fields in Splunk. Field aliases can be used to normalize data across different sources and sourcetypes that have different field names for the same concept. For example, you can create a field alias for src_ip that maps to clientip, source_address, or any other field name that represents the source IP address in different sourcetypes. Field aliases can also be used in lookup file definitions to map fields in your data to fields in the lookup file. For example, you can use a field alias for src_ip to map it to ip_address in a lookup file that contains geolocation information for IP addresses. Field alias names do not replace the original field name, but rather create a copy of the field with a different name. Field alias names are case sensitive when used as part of a search, meaning that src_ip and SRC_IP are different fields.

The correct answer is A. Field alias123.
In Splunk, a field alias is a knowledge object that you can use to assign an alternate name to a field3. This can be particularly useful when you want to normalize your data to comply with the Splunk Common Information Model (CIM)12.
The CIM provides a methodology for normalizing values to a common field name1. It acts as a search-time schema to define relationships in the event data while leaving the raw machine data intact2. By using field aliases, you can map vendor fields to common fields that are the same for each data source in a given domain4. This allows you to correlate events from different source types by normalizing these different occurrences to a common structure and naming convention1.

The transaction command is a search command that creates a single event from a group of events that share some common characteristics. The transaction command can group events based on fields, time, or both. The transaction command can also create some additional fields for each transaction, such as duration, eventcount, startime, etc. The transaction command does not group a set of transactions based on time, but rather groups a set of events into a transaction based on time. The transaction command does not separate two events based on one or more values, but rather joins multiple events based on one or more values.
The transaction command does not return the number of credit card transactions found in the event logs, but rather creates transactions from the events that match the search criteria.