C

Step 1: Understand the context of sterilizing milk. Sterilization using electromagnetic radiation aims to kill harmful bacteria and pathogens present in the milk, making it safe for consumption.
Step 2: Explore the types of electromagnetic radiation listed in the options:

• X-ray: Primarily used in medical imaging, not suitable for sterilization.
• Y-ray: Not commonly referred to in the context of milk sterilization; likely a typographical error for gamma rays, which are indeed used for sterilization but are not mentioned.
• UV rays: Commonly used for disinfection purposes, including the sterilization of milk, as they can effectively kill bacteria.
• Radiowaves: Have longer wavelengths and are not effective for sterilization.

Step 3: Conclude that the best answer is UV rays, as they are specifically effective in sterilizing milk by eliminating microbial pathogens.
Therefore, the correct option is C.