Final answer:
The ultrasound frequency with the highest rate of absorption (heating) is 3 MHz, which is used in medical applications such as ultrasound diathermy for deep-heat treatments in physical therapy.
Step-by-step explanation:
The answer to the question of which ultrasound frequency has the highest rate of absorption (heating) is 3 MHz. Higher frequencies of ultrasound, such as 3 MHz, are absorbed more rapidly by tissues and thus result in a greater rate of heating compared to lower frequencies like 1 MHz. This characteristic is utilized in medical applications, specifically in modalities such as ultrasound diathermy which is used for deep-heat treatments to relieve pain and improve flexibility in muscles. It is important for therapists to use skill to avoid potential tissue damage when applying ultrasound treatments.