Final answer:
An oscillator that operates below the audible range for humans (below 20 Hz) is termed an infrasonic oscillator, which contrasts with ultrasound used in medical diagnoses.
Step-by-step explanation:
An oscillator with a frequency below the range of human hearing which is used as a controller is called an infrasonic oscillator. The range of human hearing is generally accepted to be from 20 Hz to 20 kHz. Sounds below this range, under 20 Hz, are termed infrasound. In contrast, sounds above the human hearing range, over 20 kHz, are called ultrasound. For instance, ultrasound is commonly used in noninvasive medical diagnoses, such as fetal monitoring in the womb.
Therefore, the correct answer to the question is: 3) Infrasonic oscillator.