#### Answer

The correct answer is:
B. $25~N$

#### Work Step by Step

The two pads provide a total retarding force of 20 N. Since there are two pads, each pad provides a retarding force of 10 N. We can find the normal force $F_N$ of each pad on the wheel:
$F_N~\mu_k = 10~N$
$F_N = \frac{10~N}{\mu_k}$
$F_N = \frac{10~N}{0.4}$
$F_N = 25~N$
The correct answer is:
B. $25~N$