Answer:
c. When they are traveling at low speeds, when they are almost stopped
Explanation:
The rate at which they ions loose energy depends upon its charge and mass and not on its speed. For example, We know that alpha particles have high mass and high charge so they loose more energy than gamma radiation which has no mass and no charge. The slower heavy particle which has high linear energy transfer.
Hence, when they travel at low speed they are almost stopped.
There the correct answer is option C
