**Internal Energy:**
Internal energy is the sum of all the kinetic and potential energies of the particles that make up a system. It represents the total energy contained within a system and includes both the energy associated with the motion of particles (kinetic energy) and the energy associated with their relative positions (potential energy).

**Explanation:**

1. **Kinetic Energy:**
- Kinetic energy is the energy associated with the motion of particles.
- In a substance, the particles are in constant motion, and this motion contributes to the internal energy of the system.
- The kinetic energy of the particles depends on their mass and velocity.
- As the temperature of a substance increases, the average kinetic energy of its particles also increases.
- Therefore, a part of the internal energy is kinetic energy.

2. **Potential Energy:**
- Potential energy is the energy associated with the relative positions of particles.
- In a substance, particles are attracted to one another due to intermolecular forces.
- The arrangement and distances between particles determine the potential energy of the system.
- When particles are closer together, the potential energy is higher.
- As the temperature of a substance increases, the particles gain more thermal energy, leading to an increase in their average distance from one another.
- Therefore, a part of the internal energy is potential energy.

3. **Combination of Kinetic and Potential Energy:**
- Since internal energy is the sum of all kinetic and potential energies, it is partly potential and partly kinetic.
- The proportion of kinetic and potential energy within the internal energy depends on factors such as temperature and intermolecular forces.
- At higher temperatures, the kinetic energy of particles dominates, while at lower temperatures, the potential energy has a more significant contribution.
- The exact distribution of kinetic and potential energy varies depending on the specific conditions of the system.

Therefore, the correct answer is option 'A' - Internal energy is partly potential and partly kinetic.