Chapter 4 - Forces and Newton's Laws of Motion - Check Your Understanding - Page 100: 14

(c) is correct.

- Maximum static frictional force: $f_s^{max}=\mu_sF_N$ - Kinetic frictional force: $f_k=\mu_kF_N$ So when $\mu_s=1.4\mu_k$, it only means that $f_s^{max}=1.4f_k$, as $f_s$ changes according to the propelling force exerted on the object. So (c) is correct.