Stokes' Law governing particle sedimentation:

(A) is true: It quantifies the terminal settling velocity (\(v\)) of rigid particles in a viscous fluid under laminar flow conditions (\(\text{Re} < 0.1\)). \lt strong\gt (B) is true\lt /strong\gt : Terminal velocity is directly proportional to the square of particle radius (\(r^2\)) and the density difference between particle (\(\rho_s\)) and fluid (\(\rho_f\)).
(C) is true: It forms the theoretical basis of the hydrometer and pipette methods used to determine soil particle size distribution.
(D) is true: Stokes' law derivation mathematically assumes that particles are smooth, rigid, isolated spheres (non-spherical soil particles are thus expressed as 'equivalent spherical diameters').

Darcy's law for flow through porous media is stated as:

where:
\(q\) is the specific discharge / flux density
\(\frac{dh}{dl}\) is the hydraulic gradient \(K\) is the constant of proportionality, which is universally known as the Hydraulic Conductivity (or hydraulic permeability coefficient).