D

Stationary waves, also known as standing waves, are formed when two waves of the same frequency and amplitude travel in opposite directions and interfere with each other.
Step 1: Identify the types of media where waves can travel. Waves can propagate in solids, liquids, and gases.
Step 2: Clarify the conditions needed to produce stationary waves. These conditions are fulfilled in all three phases of matter.
Step 3: In solids, stationary waves can be formed in strings or sound waves. In liquids, they can occur in water waves, and in gases, they are seen in sound waves as well.
Conclusion: Since stationary waves can be produced in solids, liquids, and gases, the correct answer is D.