The eval command supports a number of functions that you can use in your expressions to
performcalculations, conversions, string manipulations and more2. One of the eval command functions is
tostring(), which converts a numeric value to a string value2. Therefore, option D is correct, while options A,
B and C are incorrect because they are not valid eval command functions.

The correct answer is C. 'weekly_sales (3.99, 10)'. This is because search macros accept arguments without quotation marks or dollar signs, and the number of arguments must match the number of parameters defined in the macro. The other options are incorrect because they either use quotation marks or dollar signs around the arguments, or they provide a different number of arguments than the macro expects. You can learn more about how to use search macros in searches from the Splunk documentation1.

Explanation
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