A car of mass 1500 kg is moving with $20m{s}^{-1}$ velocity. If the breaks are applied, it comes to rest in 5 s, then the retarding force is

Given,
Mass of car = 1500 kg
Initial velocity = 20m/s
Time to stop = 5s
Using first equation of motion,
v = u + at
v =0, u = 20m/s, t = 5s
Retardation, $a=-\frac{20}{5}=-4m/s^2$  
 (-ve sign show that force is applied opposite to the direction of motion)
Retarding force is given by $F_{retard}=M\left|a\right|=1500\times 4$ 
$F_{retard}=6000N$ 
Hence, option (b) is correct option.