C

To analyze the speed of ultrasonic, infrasonic, and audible waves, we start with the definitions:
- Ultrasonic waves are sound waves with frequencies above 20 kHz.
- Audible waves are sound waves with frequencies between 20 Hz and 20 kHz.
- Infrasonic waves are sound waves with frequencies below 20 Hz.

The speed of sound generally increases with frequency in a medium due to different physical properties. This can be roughly given by the inequalities:

1. $V_u \geq V_a \geq V_i$
2. $V_u \approx V_i$ for certain conditions (specifically when considering media like air at standard conditions).

Hence, option C, which suggests that $V_u \approx V_i$, is valid in many practical scenarios, making it the correct answer.