Sublimation is the process in which a substance changes directly from a solid to a gas phase without passing through the intermediate liquid phase. The sublimation curve on a P-T (pressure-temperature) diagram represents the conditions under which sublimation occurs for a particular substance.

Explanation:

Slope of the Sublimation Curve:
- The slope of the sublimation curve on a P-T diagram indicates the rate at which the substance transitions from the solid phase to the gas phase as the temperature and pressure change.
- The correct answer is option 'C', which states that the slope is positive. This means that as the temperature and pressure increase, the sublimation rate also increases.
- A positive slope indicates that the substance is more likely to undergo sublimation at higher temperatures and pressures.

Interpretation of the Slope:
- A positive slope suggests that the substance has a higher vapor pressure at higher temperatures and pressures. This means that the substance's molecules have more energy and are more likely to break free from the solid phase and enter the gas phase.
- The positive slope of the sublimation curve is consistent with the idea that sublimation is an endothermic process, meaning it requires the input of energy. As the temperature and pressure increase, the substance gains more energy, facilitating the sublimation process.
- On the other hand, a negative slope would indicate that the substance becomes less likely to undergo sublimation as the temperature and pressure increase. This would suggest that the substance has a lower vapor pressure at higher temperatures and pressures.
- A zero slope would indicate that the substance does not undergo sublimation, meaning it does not transition directly from the solid to the gas phase under any conditions.

In conclusion, the correct answer is option 'C', which states that the slope of the sublimation curve on a P-T diagram is positive. This indicates that the substance is more likely to undergo sublimation at higher temperatures and pressures, as its vapor pressure increases with increasing energy.