Total travel time gets reduced by 75%. So, total travel time becomes 1/4th of the original.

So, s’(New speed) = 4s(Original speed)

Speed of boat : Speed of river = x:1

When the speed of boat is normal

Speed Upstream = x-1

Speed Downstream = x+1

Here, the distance is constant. So, average speed can be found out by the harmonic mean of two speeds = 2ab/ (a+b)

Here, average speed will be 2(x^2-1)/2x = (x^2-1)/x

When the speed of boat is doubled

Speed upstream = 2x-1

Speed downstream = 2x+1

Here, the distance is constant. So, average speed can be found out by the harmonic mean of two speeds = 2ab/(a+b)

Here, average speed will be 2(4x^2-1)/4x = (4x^2-1)/2x

Now, we know that

New speed = 4*Original speed

So,

(4x^2-1)/2x = 4(x^2-1)/x

4x^2 – 1 = 8x^2 – 8

4x^2 = 7

x = √7/2

Ratio = x:1 = √7/2

Option (B)