The correct answer is option 1 - Cosmic rays.

Cosmic rays are high-energy particles, mainly originating from outside of our solar system. They can include protons, electrons, and atomic nuclei. Cosmic rays have the highest frequency compared to the other options listed.

X-rays, option 2, are electromagnetic waves with higher energy and frequency than visible light. They are often used in medical imaging.

Radiowaves, option 3, are electromagnetic waves with a relatively lower frequency compared to cosmic rays and X-rays. They are used for various applications like television and radio broadcasting.

Microwaves, option 4, are also electromagnetic waves with a lower frequency than X-rays but higher than radiowaves. They are used in microwave ovens and communication technology.

In summary, out of the given options, cosmic rays have the highest frequency.