What is the first step in creating and applying User Defined Functions (UDFs)?

The correct answer is defining the UDF, which is indeed the foundational first step in creating and applying User Defined Functions in a data processing environment like Databricks. Defining the UDF involves specifying the function's behavior, which includes the inputs it accepts, the logic it executes, and the output it produces. This is a crucial step because the UDF needs to be properly structured to ensure that it performs the intended operations when it is called later.

Once the UDF is defined, it can then be registered, allowing it to be recognized and utilized throughout the Databricks environment. After registration, users can apply the UDF to their datasets and test its functionality, ensuring that it operates correctly in the context of data analysis tasks. Thus, without the definition step, subsequent actions like registration, application, and testing cannot be effectively carried out. This illustrates the importance of starting with a clear and precise definition of what the UDF is intended to do.