**Explanation:**

During phase transitions such as vaporization, melting, and sublimation, certain properties of the substance change. Let's examine each of these phase transitions and their effects on pressure, temperature, volume, and entropy:

**Vaporization:**

Vaporization is the process of converting a liquid into a gas. During this phase transition:

- Pressure and temperature can vary depending on the conditions, but they are not constant. For example, in an open system, the pressure may change due to the evaporation of the liquid.
- Volume increases significantly as the liquid expands into a gas phase.
- Entropy increases as the substance transitions from a more ordered state (liquid) to a more disordered state (gas). This is because gas molecules have more freedom of movement compared to liquid molecules.

**Melting:**

Melting is the process of converting a solid into a liquid. During this phase transition:

- Pressure and temperature can vary depending on the conditions, but they are not constant. For example, the melting point of a substance is the temperature at which it changes from a solid to a liquid, and this temperature can vary depending on pressure.
- Volume generally increases as the solid expands upon melting. However, there are exceptions where the volume decreases upon melting, such as in the case of water between 0°C and 4°C.
- Entropy generally increases as the substance transitions from a more ordered state (solid) to a more disordered state (liquid). This is because the movement of particles in a liquid is more random compared to a solid.

**Sublimation:**

Sublimation is the process of converting a solid directly into a gas without going through the liquid phase. During this phase transition:

- Pressure and temperature can vary depending on the conditions, but they are not constant. For example, the sublimation point of a substance is the temperature at which it changes from a solid to a gas, and this temperature can vary depending on pressure.
- Volume increases significantly as the solid expands into a gas phase.
- Entropy increases as the substance transitions from a more ordered state (solid) to a more disordered state (gas). This is because gas molecules have more freedom of movement compared to solid molecules.

Therefore, the correct answer is option 'C' - both pressure and temperature do not remain constant during phase transitions, but volume and entropy do change.