The relationship between data models and pivots is that data models provide the datasets for pivots. Data models are collections of datasets that represent your data in a structured and hierarchical way. Data models define how your data is organized into objects and fields. Pivots are user interfaces that allow you to create data visualizations that present different aspects of a data model. Pivots let you select options from menus and forms to create charts, tables, maps, etc., without writing any SPL code. Pivots use datasets from data models as their source of data. Pivots and data models are not the same thing, as pivots are tools for visualizing data models. Pivots do not provide datasets for data models, but rather use them as inputs.
Therefore, only statement A is true about the relationship between data models and pivots.

The eval command 'if' function requires the following three arguments (in order): boolean expression, result if true, result if false. The eval command is a search command that allows you to create new fields or modify existing fields by performing calculations or transformations on them. The eval command can use various functions to perform different operations on fields. The 'if' function is one of the functions that can be used with the eval command to perform conditional evaluations on fields. The 'if' function takes three arguments: a boolean expression that evaluates to true or false, a result that will be returned if the boolean expression is true, and a result that will be returned if the boolean expression is false. The 'if' function returns one of the two results based on the evaluation of the boolean expression.

Explanation
The correct answer is B. Reusable pieces of search processing language.
The explanation is as follows:
Search macros are knowledge objects that allow you to insert chunks of SPL into other searches12.
Search macros can be any part of a search, such as an eval statement or a search term, and do not need to be a complete command12.
You can also specify whether the macro field takes any arguments and define validation expressions for them12.
Search macros can help you make your SPL searches shorter and easier to understand3.
To use a search macro in a search string, you need to put a backtick character () before and after the macro name[^1^][1]. For example, mymacro`.

Reference:
https://docs.splunk.com/Documentation/Splunk/8.0.3/Knowledge/FXSelectMethodstep

Reference: https://docs.splunk.com/Documentation/Splunk/8.0.3/Pivot/IntroductiontoPivot A pivot is a tool that allows you to create reports and dashboards using data models without writing any SPL commands2. You can use pivots to explore, filter, split and visualize your data using a graphical interface2. Pivots are designed for users who want to analyze and report on their data without having to learn the SPL syntax or the underlying structure of the data2. Therefore, option A is correct, while options B, C and D are incorrect because they are not the typical group of users who would use pivots.