Designer of the exhaust system of a water bike wants maximum acceleration. Which design would be use?
There are two main considerations. Firstly we can not use a straight pipe because there can be a chance of water entering the exhaust. So by what angle should the pipe be bent.
If you observe closely you will see that the air is entering horizontally in all the four cases. But it is changing direction before coming out. So the change in direction would be due to the force applied on the exhaust system.
The aim is to cause minimum deceleration.
So we will choose the system which has minimum change in horizontal component of momentum of gas. That will be only in the case a.