C

Step 1: Understand the principle of a cyclotron. A cyclotron is a type of particle accelerator that uses a magnetic field and an electric field to accelerate charged particles.
Step 2: Identify the type of particles that can be accelerated. Cyclotrons are designed specifically to accelerate charged particles, which means they work best with ions.
Step 3: Distinguish between positive and negative ions. Since negative ions contain electrons, which have a very small mass and are not effectively accelerated by a cyclotron's design, cyclotrons are typically used for positive ions.
Step 4: Conclusion. The cyclotron accelerates positive ions, such as protons and heavier ions (e.g., alpha particles). Therefore, the correct answer is C.