Let the speed of the man in still water be $x$ and the speed of the current be $y$. Let the total distance be $D$.

Downstream speed = $x+y$ Upstream speed = $x-y$ According to the question,

Time taken in rowing upstream = $2 \times$ Time taken in rowing downstream

Simplifying: $x+y=2x-2y$

$x=3y$ Thus, the ratio $x:y = 3:1$. Hence, option (d) is the correct answer.

Alternatively, If the time taken is halved, the speed must double, as the distance remains constant. So,

$x+y=2(x-y)$ Simplifying: $x=3y$ Thus, the ratio $x:y = 3:1$.