d
Force of friction provides the necessary centripetal force.
$f \leq \mu_{s} N=\frac{m w^{2}}{R}$
$v^{2} \leq \frac{\mu_{s} R N}{m}$
$v^{2} \leq \mu_{s} R g \quad[\because N=m g]$
$v \leq \sqrt{\mu_{s} m g}$
The maximum speed of the car in circular motion is $v_{\max }=\sqrt{\mu_{s} R g}$