Final answer:
Grandmaster Flash is credited with perfecting the technique of scratching, a pivotal innovation in rap music. He played a significant role in the development of hip hop, demonstrating the concept of stimulus diffusion as his techniques were adopted and adapted by DJs worldwide.
Step-by-step explanation:
The DJ credited with perfecting the critical rap music innovation of "scratching" is Grandmaster Flash.
Scratching, a key aspect in the creation of hip hop music, involves the rhythmic manipulation of a vinyl record on a turntable. Grandmaster Flash, whose real name is Joseph Saddler, was an influential figure in the early days of hip hop, and he is celebrated for his technical skills and the development of scratching techniques. These techniques involved the DJ moving the record back and forth on the turntable, using their hands to create a unique sound that became an essential element of rap music.
Grandmaster Flash's contributions to the genre were foundational and are still referenced and built upon by DJs and artists today. His innovative techniques and musical expressions were important in the rise of hip hop from the streets of New York to a respected and influential global phenomenon. This contribution is a perfect example of stimulus diffusion, where the core element of hip hop spread and was adapted differently by each individual who embraced it, further demonstrating the cultural impact of the art form.