ANS:C - cohesion

The shear strength in plastic undrained clay is primarily due to: Cohesion. Cohesion is the primary mechanism responsible for providing shear strength in plastic undrained clay. Unlike sands, which rely more heavily on intergranular friction for shear strength, clay particles have a tendency to stick together due to electrostatic forces and water films surrounding the particles. This cohesion between clay particles allows the clay to resist shear forces and maintain its strength, particularly in undrained conditions where there is limited dissipation of pore water pressure. While inter-granular friction and internal friction can contribute to shear strength in some soils, they are generally more applicable to granular soils like sands and gravels rather than cohesive soils like clays. Therefore, in plastic undrained clay, cohesion is the primary factor contributing to shear strength.