Final answer:
The appropriate ultrasound frequency for heating muscle 4 cm beneath the skin is 1 MHz since 0.8 to 1 MHz frequencies are typically used for deep heat treatments like ultrasound diathermy, ensuring adequate penetration and safety.
Step-by-step explanation:
To heat muscle that is located 4 cm beneath the skin using ultrasound, the most appropriate frequency to use would be 1 MHz. Frequencies of 0.8 to 1 MHz are typically used for deep-heat treatments like ultrasound diathermy. This treatment modality is applied often in athletics and physical therapy to relieve pain and improve flexibility in injured or overworked muscles. A frequency of 3 MHz, as stated in option B, could be used for shallower tissue because higher frequency ultrasound does not penetrate as deeply as lower frequencies. Therefore, for muscle tissue situated 4 cm deep, 1 MHz is the more effective choice to ensure adequate penetration without overheating the tissues, preventing damage. The skill of the therapist is crucial to avoid complications such as 'bone burns' from ultrasound diathermy treatments.