By Jurin's law, capillary rise height is inversely proportional to pore radius ($h_c = \frac{4\sigma \cos \theta}{\gamma_w d}$). Clay soils have extremely small microscopic pore channels ($d \ll 0.002\text{ mm}$), resulting in theoretical capillary rise of several meters, whereas sandy soil has large macropores resulting in capillary rise of only a few centimeters to tens of centimeters. Option analysis: (A) Capillary rise is greater in clay than sand due to smaller pore diameter. (B) Contradicts Jurin's law. (C) Pores are vastly different sizes. (D) Capillary rise occurs in both.

When floodwater rises by height $h_w$ above ground level, at any depth $z$ below the surface, the total stress increases by $\Delta \sigma = \gamma_w h_w$ and the pore water pressure increases by $\Delta u = \gamma_w h_w$. Because $\Delta \sigma = \Delta u$, effective stress $\sigma' = \sigma - u$ remains unchanged. Option analysis: (A) Equal increase in pore water pressure and total stress is the exact soil mechanics principle. (B) Pressure increases, not decreases. (C) Total stress also increases. (D) Pore pressure increases.

Derivation of the Laplace equation ($\nabla^2 h = 0$) governing steady-state 2D/3D seepage through porous media is based on four core assumptions: 1. Water is incompressible (constant fluid density $\rho$, disproving statement A); 2. Soil matrix is rigid, incompressible, and saturated porous medium with invariant void ratio (matching B); 3. Steady-state continuity condition holds where volumetric inflow equals outflow without change in internal water storage (matching C); and 4. Laminar seepage strictly obeys Darcy's law with known boundary conditions (matching D). Hence, (B), (C) and (D) only are valid assumptions. Option analysis: (A) (B), (C) and (D) only is the correct answer. (B) Incorrectly assumes water is compressible. (C) Includes false statement (A). (D) Includes false statement (A).