The correct option is (b) M\(\ddot{x}\) is time-dependent

To elaborate: The dynamic equation of motion of a structure is a 2^nd order equation. It is written as M\(\ddot{x}\) + C\(\dot{x}\) + Kx = F, where M, C and K are the mass, damping and stiffness matrices of structure, respectively. All the forces in the equation are time-dependent. M\(\ddot{x}\) is inertia force, Kx is spring force, and C\(\ddot{x}\) is damping force.