Provided sufficient water supply and connections as shown in the picture, Which tank is will be the first one to be filled up (90% filled will be considered filled). No funny assumptions. Will it be tank A, tank B, tank C or tank D?
It may be intuitive that tank D will be the first one to be filled out and then tank C, but the reality is that it will not get a drop of water till tank C is full because the water level in the connective pipe will be equal to the water level in tank C. There is video to prove this answer.
P.S. : Tank A and tank B will never be full because water will start overflowing from tank C and tank D once they both are full.