a
(a) When driver applies brakes and the car covers distance \(x\) before coming to rest, under the effect of retarding force \(F\)

then \(\frac{1}{2}m{v^2} = Fx\) 

\(⇒\) \(x = \frac{{m{v^2}}}{{2F}}\)

But when he takes turn then \(\frac{{m{v^2}}}{r} = F\)

\(⇒\)  \(r = \frac{{m{v^2}}}{F}\)

It is clear that \(x = r/2\)

i.e. by the same retarding force the car can be stopped in a less distance if the driver apply breaks. This retarding force is actually a friction force.