Answer
The only safe driveway is the driveway with an incline of $6^{\circ}$.
Work Step by Step
We can use the force equation to find the maximum angle at which it is safe to park a car. Let's assume that the force of static friction $F_f$ directed up the slope is equal in magnitude to $mg ~sin(\theta)$ which is the component of the car's weight directed down the slope:
$mg ~sin(\theta) = mg ~cos(\theta)\cdot ~\mu_s$
$tan(\theta) = \mu_s$
$\theta = tan^{-1}(\mu_s) = tan^{-1}(0.15) = 8.5^{\circ}$
The answer shows that the only safe driveway is the driveway with an incline of $6^{\circ}$ as the maximum safe angle is $8.5^{\circ}$. This means that the other two driveways are not safe because the incline is too large.
