Challenge 1: Which build.gradle File Gets a New Dependency — Solution Walkthrough

The answer:

The module-level build.gradle file — app/build.gradle, not the
project-level one.

Why, per this chapter's own explanation:

Individual library dependencies belong inside a specific module's own
dependencies block, since a dependency is something one module needs
in order to compile and run — in a typical single-module app, that's
the app module itself. The project-level build.gradle (and
settings.gradle) instead configures overall build settings and lists
which modules exist in the project at all; it has no dependencies
block of its own for a specific library to go into.

What would go wrong with the other choice:

Adding a dependency line to the project-level file wouldn't make that
library available to the app module's own code at all — Gradle
wouldn't know which specific module the dependency is meant for, since
the project-level file's job is describing the project as a whole, not
declaring what any one module needs to compile.

WHY THIS WORKS AS AN ANSWER
------------------------------
This exercise is a direct, practical application of this chapter's own
project-level vs. module-level distinction to the single most common
real task a developer performs in this file — adding a dependency —
correctly identifying which scope that specific kind of change
belongs to.