Correct option is (d) 2 to 2.5 MHz
The best explanation: For obstetric studies, ultrasonic frequency of about 2 to 2.5 MHz is usually employed, whereas in the study of blood flow in arteries and superficial blood vessels frequencies around 5–10 MHz are preferred. The level of ultrasonic energy transmitted into the body is generally kept between 10–15 mW/cm^2.