What must you include in a module to reference output values from a child module?

To reference output values from a child module in Terraform, you must include an output argument in that module. When you define an output in a module, you are essentially specifying a value that can be accessed and utilized in other modules or resources outside of that module. This enables you to encapsulate the functionality of the module while still exposing specific values that are essential for further operations.

When you define an output argument within a child module, you ensure that the values generated or processed by that module are made accessible to any module or resource that calls it. This promotes modularity and reusability since the parent module can retrieve and utilize these output values as needed, facilitating communication between different components of your infrastructure.

While the other options may relate to various aspects of module structure or hierarchy, they do not specifically pertain to how output values can be referenced. For example, having a child variable or understanding module hierarchy does not directly affect the accessibility of output values from a child module. Similarly, a parent reference indicates the relationship between modules but does not by itself allow for output value referencing.