The question presents a situation where a car is trying to catch up with a bus. The difference in their speeds is the relative speed by which the car approaches the bus, which is 50 km/hr - 30 km/hr = 20 km/hr. If it takes 15 minutes for the car to catch up to the bus at this speed, you can figure out the distance between them using the formula "time = distance/speed." Converting 15 minutes into hours (15/60 = 0.25 hours) and substituting the values into the formula, you get "0.25 hours = distance / 20km/hr", which results in a distance of 5 km.

Option 1 (5 km) is the correct answer because the calculations clearly prove it.

Option 2 (7.5 km), option 3 (12.5 km), and option 4 (15 km) are incorrect because they would require different values for either the relative speed or the time taken, which contradicts the given problem.