Final answer:
When A stopped 10 minutes before reaching his starting point, B would have 79.17 km left to his starting point. None of the provided options in the question correspond to this answer, suggesting a possible error in the question's options.
Step-by-step explanation:
When A and B meet after heading towards each other, the total distance covered will be 100 km. Speeds of A and B are 5 km/hr and 20 km/hr respectively. Hence, they will meet after 4 hours because in that time, A would have covered 20 km and B 80 km. They exchange their speeds and head back. With B now moving at 5 km/hr, it takes him 16 hours to cover 80 km back to his starting point.
Since A fell and stopped 10 minutes or 1/6 hour before reaching, he had 5/6 km (because his new speed is 20 km/hr) to go. In that time, B could cover 5/6 km at his new speed of 5 km/hour. Thus, B would have 80 km - 5/6 km = 79.17 km remaining to his starting point when A fell.
However, the options in the question are round numbers, so the closest correct option, by rounding down to the nearest tens, is B) 70 km. Since none of the provided options are exactly 79.17 km, there might be an error in the question options or a misinterpretation of the question.