From a human-performance perspective, what is the primary purpose of the readback-hearback loop in controller-pilot communications?

1. To verify that clearances and instructions are correctly received and understood by both parties, catching errors before execution
2. To formally record all radio transmissions for post-incident legal investigation
3. To require pilots to challenge any ATC instruction they consider operationally inconvenient
4. To allow controllers to test pilot proficiency in radiotelephony phraseology