The figure above represents a system of roads along which traffic flow

The questions says traffic flow is spread evenly. That means, we have to divide the traffic along the two branches equally.

Lets say we have 100% traffic, we first slipt it into two as we have two branches at Z.

The lower branch will take the complete traffic towards Y. So, 50% of the traffic is coming towards Y via lower branch.

Now, the upper branch is further divided into two. So half of what it was carrying(50%) = 25 % would be be long each of those branches of upper branch. Now , the out of these two there is a further split in which one branch is towards Y and the other towards X. Thus, X will carry only 12.5% of the traffic while Y will carry another 12.5 from X along with the earlier 50% and 25%.

Thus, total traffic Y will carry is 87.5%. Hence , answer is A.