What is the primary purpose of using a sealed class in Kotlin?

The question asks for the primary purpose of using a sealed class in Kotlin. Let's analyze the options:

• To allow unlimited extensions without constraints: This is incorrect. Sealed classes are specifically designed to restrict inheritance, not allow unlimited extensions.
• To provide a blueprint for generating random data classes: This is incorrect. Sealed classes are not used for generating random data classes.
• To restrict class inheritance to a known set of subclasses: This is the correct purpose. A sealed class allows you to define a restricted class hierarchy. All direct subclasses of a sealed class must be declared within the same file (or module, since Kotlin 1.5) as the sealed class itself. This ensures that all possible subclasses are known at compile time, which is very useful for exhaustive when expressions.
• To facilitate the creation of complex data structures using multiple inheritance: Kotlin does not support multiple inheritance for classes. Sealed classes are not related to multiple inheritance; they manage a controlled single inheritance hierarchy.

The final answer is $\boxed{\text{To restrict class inheritance to a known set of subclasses}}$.