D.Spring scales and kinematics measurements (rulers and clocks) together establish the second law and inertial mass
Like the first law, the second law involves measurements of force (using spring scales) and of acceleration (using rulers and clocks). For a nonzero net force F one finds that the acceleration (a) is along (F) , and that (a) is linearly proportional to (F). This proportionality is basiscally the second law of motion, and the proportionality constant we call 1/m.
Because (a) is measured to a part in 1044 but (F) is measured to a part in 1033, we can determine the inertial mass (m) to a part in 1033 via the measured slope of a plot of (a) vs (F).