Answer: B) the car was at the park because the soil in the tires matched that from the park, and the child was in the car because the child's hair was in the car.
The Locard's principle of mutual exchange suggests that when an object comes in contact with a surface or another object, it leaves it's trace. The phrase " Contact leaves traces " is ideal to define this principle. The trace can be useful for the establishing the relation of criminal or victim with the object or surface in contact.
In the given situation, according to the Locard's principle of mutual exchange it can be concluded that the car was the same that was observed by the witness in the park as it exhibit evidences which suggests that the car was present in the park. These evidences includes soil in the tires which can be matched with the soil of the park, and the hair sample of the child obtained which can be matched with that of parents with the DNA analysis, proving the fact that the child abducted was present in the car. Therefore, Locard's principle was useful in this case for linking the car and it's owners with the crime of abduction and establishing the identity of the abducted child.