correct answer is : d) 4 km/ hour

Explanation

According the question,
B’s speed 8 km/hour.
Here, B overtakes A in 15 minutes or \frac{15}{60} hour \rightarrow \frac{1}{4} hour \left[convert\ min\ to\ hr\right]
\therefore Total distance covered by B = 8 km/hr \times \frac{1}{4} hr \left[distance=speed\times t i m e\right]
= 2 km.
So that, here we understand that they total traveled 2 km.

In this question,
B starts when A has gone 1 km and overtakes A at 15 minutes.
\therefore A take 15 minutes to travel 1 km.
\therefore A take (15 \times 2) = 30 minutes or \frac{30}{60} hour \rightarrow \frac{1}{2} hour to travel 2 km.
\therefore A’s speed is = \left(2km\ \div\ \frac{1}{2}\ \right) hour \left[speed\ =\ distance\ \div\ time\right]
= 2 km \times \frac{2}{1} hr
= 4 km/hr.
Ans: A’s speed 4 km/hour.