Final answer:
Sound waves cannot travel in a vacuum because they are mechanical waves and need a medium to propagate, unlike light waves and radio waves, which are electromagnetic waves and can travel through a vacuum.
Step-by-step explanation:
The question you are asking is related to the propagation of different types of waves in a vacuum. Sound waves are mechanical waves, which means they require a medium such as air, liquid, or solid to travel through. This is because sound waves are pressure disturbances. On the other hand, light waves and radio waves are both forms of electromagnetic radiation, which do not require a medium to travel. They can propagate through a vacuum because electromagnetic fields generate each other, allowing these waves to move through empty space. Thus, the correct answer to the question 'Which of the following cannot travel in a vacuum?' is B) a sound wave.