Which of the following is illegal to park on?

Parking on a sidewalk is illegal because sidewalks are designated for pedestrian traffic. When a vehicle is parked on a sidewalk, it obstructs the path for pedestrians, which can pose safety hazards, especially for those with disabilities, children, or elderly individuals. Sidewalks provide a necessary space for people to walk safely away from vehicle traffic, and blocking them can force pedestrians to walk into the street, increasing the risk of accidents.

In contrast, parking in front of a grocery store is generally permissible as long as it's in a designated parking area and conforms to any posted regulations. Parking on the side of the road can also be legal, depending on local laws and road signage. Paid parking areas are specifically set up for vehicles and usually provide designated spots for parking, making it legal to park there as long as the required fees are paid.