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

Explanation
The correct answer is A. | where 10yearAnnerversary=Renewal-MonthYear.
The where command is used to filter the search results based on an expression that evaluates to true or false.
The where command can compare two fields, two values, or a field and a value. The where command can also use functions, operators, and wildcards to create complex expressions1.
The syntax for the where command is:
| where <expression>
The expression can be a comparison, a calculation, a logical operation, or a combination of these. The expression must evaluate to true or false for each event.
To compare two fields with the where command, you need to use the field names without any quotation marks. For example, if you want to find events where the values for the 10yearAnnerversary field match the values for the Renewal-MonthYear field, you can use the following syntax:
| where 10yearAnnerversary=Renewal-MonthYear
This will return only the events where the two fields have the same value.
The other options are not correct because they use quotation marks around the field names, which will cause the where command to interpret them as string values instead of field names. For example, if you use:
| where '10yearAnnerversary'='Renewal-MonthYear'
This will return no events because there are no events where the string value '10yearAnnerversary' is equal to the string value 'Renewal-MonthYear'.
References:
where command usage

Explanation
The correct answer is C. When events contain unstructured data.
The regular expression method works best with unstructured event data, such as log files or text messages, where the fields are not separated by a common delimiter, such as a comma or space1. You select a sample event and highlight one or more fields to extract from that event, and the field extractor generates a regular expression that matches similar events in your dataset and extracts the fields from them1. The regular expression method provides several tools for testing and refining the accuracy of the regular expression. It also allows you to manually edit the regular expression1.
The delimiters method is designed for structured event data: data from files with headers, where all of the fields in the events are separated by a common delimiter, such as a comma or space1. You select a sample event, identify the delimiter, and then rename the fields that the field extractor finds1. This method is simpler and faster than the regular expression method, but it may not work well with complex or irregular data formats1.
Reference:
1: Build field extractions with the field extractor - Splunk Documentation