Understanding Dupuit's Assumptions
Dupuit's assumptions are foundational in groundwater hydraulics, particularly in the analysis of unconfined aquifers. The correct answer, option 'D', indicates that these assumptions are primarily valid for unconfined aquifers due to specific characteristics that distinguish them from other types of aquifers.

Characteristics of Unconfined Aquifers
- **Water Table**: In unconfined aquifers, the upper surface is the water table, which can fluctuate based on precipitation and withdrawal.
- **Hydraulic Conductivity**: Dupuit’s assumptions rely on the isotropy and homogeneity of hydraulic conductivity, which is often more consistent in unconfined aquifers.
- **Flow Direction**: The flow is primarily horizontal, making the assumptions about flow paths and gradients more applicable.

Limitations with Other Aquifers
- **Artesian Aquifer**: In confined aquifers, pressure can cause water to rise above the aquifer level, violating Dupuit's assumptions of hydrostatic pressure distribution.
- **Confined Aquifer**: These aquifers are constrained by impermeable layers, which complicates the flow dynamics and contradicts the basic flow assumptions made by Dupuit.
- **Leaky Aquifer**: Although they have characteristics of both confined and unconfined systems, the leakage complicates the flow and pressure relationships, making Dupuit’s assumptions less applicable.

Conclusion
In summary, Dupuit’s assumptions hold true for unconfined aquifers due to their simpler flow dynamics, uniform hydraulic properties, and direct relationship between hydraulic heads and flow. This makes them a reliable basis for groundwater analysis in civil engineering applications.