a) factor denoting the acceleration response spectrum of the structure subjected to earthquake ground vibrations
b) factor by which the actual base shear force is reduced
c) factor to obtain the design spectrum
d) factor used to obtain the design seismic force

Answer: a
Explanation: Structural response factor (S_a/g) is the factor denoting the acceleration response spectrum of the structure subjected to earthquake ground vibrations. It depends on natural period of vibration and damping of the structure. Response Reduction Factor(R) is the factor by which the actual base shear force is reduced. Zone Factor(Z) is factor to obtain the design spectrum. Importance Factor (I) is the factor used to obtain the design seismic force.