A takes 2 hours more than B to walk d km, but if A doubles his speed, then he can make it in 1 hour less than B. How much times does B required for walking d km ?

Answer: Option C

Suppose B takes x hours to walk d km
Then, A takes (x + 2) hours to walk d km
A's speed = $$\left(\frac{d}{x+2}\right)$$  km/hr and
B's speed = $$\left(\frac{d}{x}\right)$$  km/hr
A's new speed = $$\left(\frac{2d}{x+2}\right)$$  km/hr
$$\begin{array}{rl}& \therefore \frac{d}{\left(\frac{d}{x}\right)}-\frac{d}{\left(\frac{2d}{x+2}\right)}=1\\ & \iff x-\left(\frac{x+2}{2}\right)=1\\ & \iff x-2=2\\ & \iff x=4\text{ hours}\end{array}$$