Bats use ultrasonic waves to detect obstacles in their flying path. Ultrasonic waves are sound waves with frequencies higher than the upper audible limit of human hearing. Bats emit these ultrasonic waves and listen for the echoes that bounce back when they encounter objects. By analyzing the time taken for the echo to return and the changes in frequency, bats can accurately determine the distance, size, and shape of objects in their environment.

Option 1: Infrared waves are a type of electromagnetic wave with longer wavelengths than visible light. While some animals, like snakes, use infrared waves for navigation and hunting, bats primarily rely on ultrasonic waves.

Option 2: Electromagnetic waves encompass a wide range of waves, including visible light, radio waves, and microwaves. Bats do not use electromagnetic waves in general to detect obstacles, specifically.

Option 4: Radio waves have even longer wavelengths than infrared waves and are commonly used for communication purposes. Bats do not use radio waves to detect obstacles in their flying path.

In conclusion, the correct answer is option 3: Ultrasonic waves.