C

Step 1: Understand the characteristics of X-rays. X-rays are classified into two main types based on their energy and penetrating ability: soft X-rays and hard X-rays.
Step 2: Soft X-rays are lower in energy and are more easily absorbed by matter, which means they have a shorter penetrating distance in substances.
Step 3: On the other hand, hard X-rays have higher energy and greater penetrating power, allowing them to travel through longer distances within various materials.
Step 4: Continuous X-rays refer to a broad spectrum of X-ray radiation produced, but they are not specifically classified based on their penetrating ability.
Conclusion: Therefore, the X-rays that can penetrate through longer distances in substances are correctly referred to as Hard X-rays.